Why is it omitted? In the Rite formerly used in France there were times when the Benedictus was omtted : Because the Sanctus and Benedictus were separated by the Consecration, the custom arose of replacing the Benedictus with an 'Elevation' - the Pie Jesu is the Elevation motet which replaced the Benedictus at Requiems. Franck's Panis Angelicus was originally an Elevation as well, for Corpus Christi, I think. This is a custom in France that goes back to the Middle Ages (the chant Ave Verum was orginally sung as an Elevation. The thing is, it didn't matter too much since the priest recited the whole sanctus text at the end of the preface anyway.

What to do? I would consider three options :

1 : Benedictus from Mass XVIII, with or without Faure's Osananna.

2 : I'm not too familiar with the complete oeuvre of Faure, but you could substitute a Benedictus and Osanna from another Faure Mass (transposed if needed/possible), it's not uncommon for the Sanctus and Benedictus of the same mass to have different Osannas. The Messe Basse might work (organ/strings double the parts, ad lib).

3 : If you're a composer or know one (try a university professor or student), you could compose/commission a Faure 'pastiche' for the missing Benedictus, which would resume with a da capo of Faure's Osanna.

Very helpful indeed. Now I can proceed with some confidence. Many thanks!