It has a Eaton model 11 hydro in there ( 1100-04) They were very common on garden tractors of that era ( simplicity, ariens, cushman, mtd, etc. shouldn't be to hard to find used parts. there were different variations to watch for ( rotation and available fittings). they are quite simple and easy to work on.
The drive coupling between the hydrostat and the transaxle was the most common failure! make sure the splines are good on both the hydro output and the transaxle input shafts!
Kubota apparently have one in stock in Toronto for 2563 dollars! bet they would be happy for you to take it off their hands! LOL
http://www.stripmine.org/sears/engine/eaton_11.pdf