Evan,
There aren't many parts still available for the 915, 920 or 940 transaxles. The 930s were quite popular, with many parts still available, and even a few transaxles, but the MST 200-Series was developed to replace the 930s, so if you're using a 930, the right MST will be a bolt-in replacement, with a gear-drive Reverse instead of sprockets-&-chain.
Respectfully,
Rodney