What do you mean by "the traditional one"? The Kunc? Plainsong? I found a Kunc accompaniment here: https://archive.ccwatershed.org/media/pdfs/15/03/29/01-55-48_0.pdf For the plainsong, I've found a "lead sheet" (melody and chord symbols) on MuseScore: https://musescore.com/song/christus_vincit-6519625?srsltid=AYCX9vKyaiyRK0ZtAe-HDMp0K4hXnab-Hh_zhfk2v_6J2by3QnQF2J0T To answer your first question second, I think it's perfectly fine to accompany it on organ if they need a boost, especially the Kunc. I know others may disagree on organ accompaniment of chant, but I'd rather have it done well with accompaniment than poorly without it.