Consider purchasing a Viscount Cantorum Duo. I know of two churches near me that have purchased them and are both pleased with them. You won't have issues getting it into the loft, since the complete organ contains several pieces (unit with manuals, separate pedalboard, expression pedals, bench, and stand), and while it has its own built-in speakers, you can also connect it to whatever PA or sound system you have at hand, to make it fully present in a large space. It costs around eight grand — I'm not sure if that qualifies as a "cheap substitute," but it's a reasonably good substitute for 6-8 months.

I have a "Johannus One" keyboard - I believe it is basically the same as the Dexibel Which Earl-Grey mentioned above. Plugged into an external monitor or two, it works great for a rehearsal space, outdoor services, temporary situations, and even softly accompanying chant or polyphonic works. It almost comes off like a continuo instrument. Highly recommend! You can purchase from your local Rodgers dealer.

I would not want to have anything other than an organ (with pedals) for any length of time. You can’t play rep in any serious way on a keyboard, and you can’t play hymns or accompany except in the most rudimentary and obnoxious way.

The weight of a low-end digital organ console (with pedalboard removed) is less than that of an upright piano. Any piano-moving company would be able to move it up the stairs and would be very happy to have a break from moving pianos up and down stairs.

I would look for a cheap or free used Allen or Rodgers nearby, on Facebook (Marketplace or Electronic Organ Garage Sale). Any post-73 Allen with stoptabs (drawknob consoles are heavier and harder to move), or any 90s or newer Rodgers (older Rodgers are analog and many weigh 600+lbs). I don’t know the size of the church, but most smaller Allens can be wired to play through just 2-4 channels/speakers if the internal speakers aren’t beefy enough. Or if you really want to be cheap – feed the headphone out into a couple keyboard amps.

Say you might spend $1500k on a used 30yo organ. It would probably be not more than $500 to move it in-town and up a flight of stairs. If you can’t handle the setup yourself, maybe another $250 to get the local Allen/Rodgers tech to get it going.

At the end of the project, you can resell for the same price and thus be out only $1,250 for organ music for the duration of the project. To me that would be a reasonable expense to have suitable music for the sacred liturgy and in the grand scheme of an organ project, it’s pennies.

If the used market isn’t great could also see if you can borrow or rent or buy a used Roland C-330.

https://www.roland.com/global/products/c-330/

The thing is extremely compact and comes apart for transport. It’s Rodgers samples and sounds pretty decent for being basically an IKEA organ. It can be plugged into a keyboard amp or other sound system if the internal speakers aren’t enough.

Content and Johannus also make some cheap organ-shaped objects at the lowest ends of their ranges which I ordinarily wouldn’t recommend – but being made of chipboard, they are easy to transport.

ghmus7:
I think you are on the right track. I suppose you want a 76 note midi keyboard rather than a 60 note one is so you can play some deeper bass notes. You can find used ones on eBay or other such sites, just making sure the keyboard has standard MIDI output. Touch and quality depends on how much you are willing to pay.

You would need a MIDI interface to convert the MIDI output from your keyboard to USB so you can plug it into the USB port on your laptop/desktop computer. Roland makes reliable ones and not expensive.

The computer should be running Windows, with plenty of RAM and here is the catch. RAM prices have been skyrocketing over the past several months because of all this AI obsession, while manufacturers have been shifting their production to DDR5 memory for this, creating a shortage of the very common DDR4 and therefore a price rise. If you are going to use a decent organ sample set, the minimum is 16gb of RAM, with 32gb ideal for larger sample sets.

You should have a SSD hard drive preferably NvMe rather than the much slower SATA. Your CPU needs to be fast with a minimum of 6 cores. AMD or Intel are both OK, although AMD seems to be better for this purpose in my experience.

As for software, GrandOrgue is free, and free sample sets are available for it. Rule Hauptwerk out because of the current pricing. Jorgan is also free and fine, but complicated to set up at first.

As for the amplifier, which would plug into the computer headphones jack with appropriate cable, there are used stereo multimedia speaker systems with subwoofer around. Maybe someone would be willing to lend their living room stereo setup for this! Used stereo guitar amps are another possibility, but many have low volume output. Perhaps there is a mic input in the loft if there is a speaker system in the church, but do not expect much bass from that.

There's also this.

https://sacredmusiclibrary.com/choir-organ

Thanks, both. I'm aware of a number of commercial options available on the market, but I think it would be game-changing (on a par with the CMAA's scans of the Liber, Graduale, Gregorian Missal etc) to have an open source version of this somewhere that works on a very average laptop. The CMAA's scans eliminated all cost barriers to the adoption of Gregorian propers anywhere; I would like to see a situation where any cost and logistical barriers to accompanying the congregation on what at least sounds somewhat like an organ are removed and musicians can access a quick and easy solution whenever they're saddled with a ghastly keyboard or electric piano!

Thanks too, Gamba...

These are all excellent solutions, but they're not what I'd call 'plug and play' at this stage.

I'm happy to work with anyone (via private message) to see what might be possible.

In brief, I think it should be possible to plug into your average electric piano and have:
1. A set of Swell divisionals, moving from a simple flute through to something a bit richer and warmer, but only with a view to accompaniment of a cantor or choir.
2. A set of Great divisionals building from piano to forte.
3. Auto pedal by default for the Great divisionals, adding weight as the dynamics build upwards.
4. A reversible 'auto pedal' piston in case you need to take it off the Great divisionals or add it to the Swell divisionals.

The Swell should be set to a lower volume than the Great. No need for a crescendo pedal - redundant in most simple accompanimental situations. No need for Swell to Great either.

That would seem to cover most eventualities in a simple context, wouldn't it? An accompaniment workhorse with repertoire taking a back seat. Not exactly something that is going to get much air time at an AGO convention, but saves one from the banalities of the electric piano's tone bank... or worse!

Folks here may be interested in my portable organ setup. I use a 61 note foldable keyboard (88 notes also available) that neatly folds into a little carrying case and quite light. I have a Dell gamer laptop with plenty of RAM, and a JBL Charge 4 Bluetooth portable speaker (more pwerful ones available), which is quite small, but really sends out the music including the bass, quite an amazing device. It is a setup that is very easy to carry around and fast to set up.

As for the software, I use the free GrandOrgue for which some nice organ samples are available also for free. My foldable keyboard is made by Donner, but other manufacturers make them, but just make sure they have some kind of MIDI capability. The keyboard does not have a standard MIDI output, but connects to the computer through USB, or even Bluetooth, since it can be battery powered; I do not use it because of the inherent latency (delay) of Bluetooth. Because the MIDI is not standard I use an Open Source software interface called LoopMidi so GrandOrgue can recognise the MIDI channels. I also do not use Bluetooth for the JBL portable speaker because of the inherent latency involved, even of Bluetooth 5.3, so I plug it directly into the headphones jack on the computer. However, Bluetooth latency may not bother some.

Ahhh … the simulacrum argument rises again…

I would not want to have anything other than an organ (with pedals) for any length of time. You can’t play rep in any serious way on a keyboard, and you can’t play hymns or accompany except in the most rudimentary and obnoxious way.

So true. I sometimes travel to small parishes in surrounding towns/villages to play for funerals, assured there is something suitable to play on, only to encounter a rudimentary Casio keyboard or something similar. It's very hard to make that sound good. I know I'm blessed at my regular parish, where I have access to both a 7" grand piano and a well-maintained vintage pipe organ, but it's disheartening that these small parishes can't even afford a decent basic digital piano.

Unpopular opinion, but do you need a temporary organ in the interim? Could you just sing a cappella?

That would be my choice. We had our grand piano removed a few years ago to install new tuning pins, so I was without a piano for a month or so while the work was being done. The cantor I normally work with for one Mass is unable to negotiate the stairs to the loft where the organ is, so we dragged a Yamaha upright from another part of the church to fill the void. I wish we had just skipped it - it didn't fill the void properly even with it being a full upright piano, and didn't have the richness of the regular piano. In retrospect, I would have done exactly what you're suggesting.