This is difficult.
There is no sacred liturgical piano repertory, so one would want to choose from literature that doesn't have too secular an air about it. It may be that some selections from Bach's WTC would make decent preludes, offertories, or even postludes. Ditto certain well considered movements from the partitas and suites, but much from the WTC would bear some resemblance to some sacred organ literature and have a certain ecclesiastical gravitas. I would avoid sonatas of the classical and romantic eras because, beautiful as they are, they have an unmistakable secular ethos. On the other hand, it may be that some well thought out slow movements from Mozart or Haydn sonatas could provide a meditative air during communion (if you have communion). Definitely avoid anything that has a 'concert' or overtly 'secular' air about it. It occurs to me that even some of Bartok's piano pieces might work well as postludes or preludes.

Too, you might find that canzonas by Buxtehude, Frescobaldi, and others, though they are manuals-only organ music, could be well played on the piano. And, you could consider some of the manualiter settings from Bach's Clavier-Ubung III. These would be nice. Also, look into manuals-only chorale based literature by Walther, Pachelbel, and others, who have written very suitable chorale variations and partitas.

There is much manuals-only organ music available, English voluntaries of the Tudor and Stuart eras, and other similar continental organ music that could work well on the piano in such circumstances.

It's really no fun playing piano for church. I had to do it once for several months whilst the nave was being redone. How happy I was when we were back 'in church' and out of the parish hall!

As for 'just playing the 4 part' version of the hymns. This can be awfully boring on the piano - but avoid the temptation to 'flesh it out' too much, lest you wander into an 'un-churchy' mode of playing. Vary the harmonies. Thicken the chordal texture and add passing notes and appogiaturas, etc. as in a chorale partita (so long as you add interest but don't throw the congregation off), but don't wander into that tasteless overtly flamboyant and gaudy pianistical manner that one has (regrettably) heard in some Protestant circles.

You didn't say anything about your particular Methodists. There is, I think, quite a variety of 'styles' of worship in this denomination. Some are hardly different from Baptists or Pentecostals. Others, high church Methodists, think nothing of aping the Anglicans, even high Anglicans. One such congregation here in Houston has one of the finest choirs in the city and regularly offers an admirably done evensong right from the BCP. This is St Paul's, Houston's most magnificent Gothic pile, located just across from the Museum of Fine Arts.

Fugues (many of them) by Johann Baptiste Peyer.

For preludes, I would look for music by Carol Tornquist, especially hymn arrangements. Also Mark Hayes. Though some of his arrangements are a little more "pop" oriented, they are all well-thought out.

Augsburg Fortress published a 10 volume set of piano arrangements for every hymn (except Taize pieces) in ELW. There should be quite a lot of hymns in common.

Also, I would recommend against Bach ... on piano.

Glen Gould would disagree with you. I would too, but his opinion is more valuable then mine.

My recommendation is nothing more than an opinion, but it is my opinion:
For a traditional-music-leaning Protestant church that has a piano instead of an organ, stick to Baroque and early Classical, including plenty of music written for harpsichord and clavier. Once you get to Romantic piano, the music becomes nearly intolerable for use in church.

I think it is not for nothing that a lot of Baroque music was written as accompaniment to some other thing - whether dances or coronations or fireworks or rich people sitting around gossiping. This makes it well-suited (often) to the needs of liturgy and also especially preludes and postludes. The "stop what you are doing and listen to this important music from MY TORTURED SOUL" music of the Romantic period is (often) not a helpful accompaniment to the praying, preparation, and quiet socializing that happens before and after a service.

JL, Adam, and Bach on the Piano.

Well, Gould! Gould was a unique genius whose Bach on the piano won him wide acclaim, in which on some days I share, and other days I don't. I, for one, variously find his Bach a) fascinating, or b) a tiring and ill-tutored presumption, and even, at times, c) amusing. His Bach was a thing unto itself, highly idiosyncratic, and unquestionably more Gould than Bach. Some others sought or seek a more respectful relationship with Bach. Turek comes to mind, as does Sir Andras Schiff - there are many. Most of the great pianists count and have counted fluency in Bach as the foundation of their art.

As self referential offerings many of these can rightly be said to be artistry of the highest order, some of which may even reveal facets of Bach that would not otherwise have been noticed. The key words here, though, are self referential - as opposed to 'Bach-referential'.

I am reminded of a lecture I attended some years ago by a Dutch harpsichord scholar and builder. This was one of the most erudite evenings in my memory. At one point he brought up the harpsichord-piano conundrum and the oft proferred non sequitur that if Bach had had a piano he would have used it. He rounded this off with the patent observation that, 'yes, and if Caesar had had cruise missiles he would have used them'. All these rather ill-conceived assertions conveniently fail to take note of the fact that Caesar didn't, nor did Bach. He didn't. Bach's keyboard oevre would certainly have been different, very different, had he been writing for the piano. It might have resembled Carl Phillip Emmanuel's Prussian sonatas or some such (we will never know), but it would have had a pianistic structure and character. This much may be put forth with certitude, for Bach was far too great a musical mind to fail to write music that would exploit the character of the particular instrument for which he was writing. The WTC, the suites and partitas, the toccatas, the Italian concerto, the chromatic fantasie, and all the rest are and were conceived in Bach's mind as harpsichord music, They are not pianistic.

All of us know that Bach played a piano on at least one occasion in the presence of Frederick the Great at Sans Souci. We also know that he wasn't particularly enamoured of it. If he had been pleased with it he no doubt would have written some very different music for it.

'They are not pianistic', I said just above. This will not keep me and many of us from playing these works on the piano and delighting in making them sound self-referentially beautiful; but, with reference to Bach's conceptual mind and the sound world of his time a good many of us will not fail to appreciate that Bach on the piano can be beautiful, but it isn't really Bach - even though we may 'like it' it doesn't sound like Bach. With a tonal pallet fundamentally different from anything Bach would have conceived of it warps the textures and inevitably introduces expressive elements that Bach would have thought rather strange for the music that he wrote.

Good composers of hymns on the piano - Isaac Watts, Charles Wesley, etc. Most old hymns sound good on a piano. Buy the piano accompaniment book for your Methodist Hymnal or learn to improvise off the guitar chords in the guitar accompaniment book. If such are not available, buy the piano accompaniment book to the hymnal Worship from GIA which should have 85% of the hymns you are interested in.

Check out Braithwaite stuff for communion. It's music for "after consecration"