There was a harmonized version of the Sanctus and Agnus of XVIII (in the context of the composite "chant mass") at Hagerstown. But it was in Spanish.

CCWatershed has an extensive collection of organ harmonizations of the Kyriale and other chants. I went through some volumes and collected a few different settings of Mass XVIII.

Though they're meant as organ accompaniments for unison chant-singing, they can also be used as choral settings — they're all written strictly for four voices. I've programmed Mass VIII (de Angelis) from these collections, and while it's a little tricky at first to line up all the syllables, it's definitely doable and effective.

Here are links to my 4-part arrangements which you are welcome to use:
Kyrie
https://drive.google.com/file/d/1gmSS705jjp9YKTd2VUkdNhvsu1th0iUm/view?usp=drive_link
Sanctus/Memorial Acclamation
https://drive.google.com/file/d/1yzdqzN5ul5bDRwaVAG4VODiI13K_PB-Q/view?usp=drive_link
Agnus Dei
https://drive.google.com/file/d/1-ayqLSdcJEV4yUZsVjQCYrle1A8ojVfv/view?usp=drive_link