Did you perhaps mean to type, "Latin Hymn Translation is Fun!"

I feel like a good deal of "Ad Mortem Festinamus" is lost without the original's Seussian sense of alliteration, metre, and rhyme.

Refrain:
We're at death's door, so sin no more!

I thought that I would write about contempt of worldly glory,
to spare from vain delights all those who'd listen to my story:
It's time to rise from wicked sleep: for here comes death most gory!

Short life is shortly over, and, in short, is nearly through now,
Here hurries no-one's lover, Death! Despair! What can you can do now?
Death puts out all the lights and pities none, not even you, now!

Unless you are converted and become quite little boy-like,
Unless your life's diverted, and your acts more good-employ-like,
You will remain perverted and quite un-Heaven's-joys-like.

When trumpet trumps & dawns the day when days all have their ending,
Then part the clouds & thereupon will sit the Judge Unbending,
Elect to rest and reprobate to hell infernal sending.

[ . . . ]

:-)

O translators of ancient words obscure,
Of musical origin, they.
To seek vernacular rhythm demure,
Recast them to music, they say.

I find it somewhat amusing that your friend translated ancient songs (already set to neums) and, turning to you, declares, "now set this to music" :-)

I'd say you have many options. You could look to the original notation for some inspiration or consider more well-known hymns that are similar. For example, the sense I get of the verse beginning, "Eia pietatis occulo placato" is not too far from "Eia ergo advocata nostra illos tuos misericordes oculos ad nos converte" of the Salve Regina. Or in "Ad mortem festinamus", the verse "Tuba cum sonuerit dies erit extrema" strikes me as similar to "Tuba mirum spargens sonum" of the Dies Irae.

So, if I was tasked to put these to music, I would look to some more well-known pieces and consider how other composers have expressed those words--not to plagiarize, but to get a sense of the mood. Then the task becomes an original expression in music of what the words mean ... your own translation, as it were.