I can't find that transmission part number. Here is a list of Peerless 204 transmission numbers:
http://www.landscapepower.com/peerless/204-SERIES/The Peerless transmission manual on the Outdoorking site (Section 11, page 63) describes a set of tests you should perform on their hydro drive before you decide the transmission is trashed. Its title is TECUMSEH / PEERLESS MOTION DRIVE SYSTEM
TRANSMISSIONS / TRANSAXLES /DIFFERENTIALS / RIGHT
It doesn't tell you how to repair anything, but it can be useful in figuring out what is wrong.