Hi, Andrew, I am nt sure if either of these is the one you are looking for, but have a look and let us know, okay? I am assuming you don't mean the one by Simon and Garfunkel.The song I found associated with Marta was called Homeward Bound, like the S&G song, but it had a different arranger and there was no midi.
Anyway, here are two maybe likely candidates:
Homeward Bound - 1917 song also popular during WWI. You'll need to scroll down to find the midi at the end of the article which includes a picture of the sheet music cover.
Hope these help.
kat