the book by Wolff (op. cit. infra) has a facsimile of the first page of the Stadler score (the opening of the Domine Jesu), which is used to highlight one modification (actually an improvement of the trombone parts) presumably made when Süssmayr incorporated the Offertorium in the presentation copy for Graf Franz von Walsegg.
I don't know off the top of my head if there is a full facsimile of Stadler's score readily available anywhere, aside from finding the library reference and ordering a microfiche if one has been made. (This reminds me, I really need some time to chase up a few facts regarding this and other questions regarding the provenance of the scores.)
I doubt anyone has attempted to score the movement to make a thorough note-by-note comparison with the Süssmayr completion, but it could be worthwhile research.
Source: Wolff, Christoph, translated by Mary Whittall; Mozart’s Requiem: Historical and Analytical Studies; Documents; Score. University of California Press, Berkeley, 1994.