I was looking for a modern attempt at 15th century fauxbourdon based on this Kyrie that I seem to have lost on my computer. We use Mass IV a lot for feasts that fall after Paschal Time ends when we have the last dash of high Masses before taking July off from any weeknight Masses. My pastor also wishes to have alternation with polyphony particularly for episcopal visits. So it's extremely helpful to have authentic repertoire; there's the Bartolucci, but it's hard, and he doesn't set all of the Gregorian masses needed. (This one is for more voices than we can realistically manage, and he omits I, so in time we'll do Isaac.)

Anyway Corpus Christi Watershed had this which I dug up via the Wayback Machine, and I put a version together to do what we need to do here. The arrangement of chant and polyphony works for us. It might not work for you, and that's okay, but hear me out (and my pastor agrees): the people might be confused, but keeping their parts in the same place is important. We are proud of congregational singing here, and so is the bishop, but if we do all-polyphony (which is also admirable for the right occasion), he won't hear it!

I can't comment on the ficta because they were added already. Note values halved from what Jeff O originally posted. I deleted all of the slurs in the polyphony since either the beams cover it or it's not entirely clear (and Trent 90 is a large source, which I can't dig through, but I'lll certainly take feedback if someone does check.)

The mensuration seems to change in the repetition of Kyrie, and it's decidedly "off-beat" by our standard, just FYI.

Video here.