I've had the Winchester Troper "Kyrie" on repeat in the car for a few days now and I still can't say I've got the thing memorized. There are so many notes, so much stepwise motion, so few syllables to help remember what comes next - I am not at all surprised that this style of music required the invention of a system of notation. Nor does the form of the earliest notation surprise me - I find myself thinking that just a wavy line to help me remember if the next step is up or down or on a new syllable would be perfect.
I'm also making no progress in directly adapting the tune to the Nam languens poem. The antiphonal verses are so different than the "Kyrie" itself - it works for that song, as structured, but it really doesn't work for a cohesive single poem. And the music of just the "Kyrie" proper is just too short to stretch over eight lines.
So I really need to tuck the "Kyrie" into the back of my head as inspiration and just compose new music.