Sorry, I don't have this one. I have Moussorgskyana, "A Selection of Choice Melodies from the works of Modest Moussorgsky," also arranged by Otto Langey in 1918; and it does identify the source of the themes. So if you can find the arrangement, you're probably all set. A quick check shows that it's also not at the University of North Texas or the University of Colorado, two archives I find easy to work with.
I do see it at UCLA in California: if you're serious, you could call them up; I haven't tried to access these materials before. I'm sure there's an access/copying fee, but from 1918 it's in the public domain so at least the paperwork isn't onerous.