My organists can only play from modern notation sheet music -- they can't play from Chant notation (square notes).
My older organist can chord a song (figure out the chords) himself -- but he needs sheet music to start from.

All I can find for this song is the square note "chant" notation. Does anyone have a modern notation Liber Usualis scan?

htt p://www.chantcd.com/samples/auctor.wma

Here is the song:
Auctor Beate Saeculi
http://www.chantcd.com/lyrics/auctor_beate_saeculi.htm

It's a Sacred Heart of Jesus -themed chant hymn.

1. Auctor beate saeculi,
Christe Redemptor omnium,
Lumen Patris de lumine,
Deusque verus de Deo.
2. Amor coegit te tuus
Mortale corpus sumere,
Ut novus Adam redderes
Quod vetus ill(e) abstulerat.
3. Ill(e) amor almus artifex
Terrae, marisqu(e) et siderum,
Errata patrum miserans,
Et nostra rumpens vincula.
4. Non corde discedat tuo
Vis ill(a) amoris inclyti:
Hoc fonte gentes hauriant
Remissionis gratiam.
5. Percuss(um) ad hoc est lancea,
Passumqu(e) ad hoc est vulnera,
Ut nos lavaret sordibus
Unda fluent(e) et sanguine.
6. Iesu tibi sit gloria,
Qui Corde fundis gratiam,
cuм Patr(e) et almo Spiritu
In sempiterna saecula. Amen.

Ok, done. Gregorian chant in modern notation looks awful most of the time because they're fairly incompatible; One predates the other by about 700-800 years. But an attempt was made! 

Hope your Pentecost went well, Matthew.