I was unable to locate a used HST I ended up buying one from Messicks
https://www.messicks.com/part/k2561-14002/hst-assy
My issue was a worn pto select arm which was slipping past rather than pushing the selector gear, see pictures. I was able to build the select arm up with weld material and grind it back into its original shape. Removing the select arm requires removing the HST and front transaxle cover and most of the gears and shafts inside. It was not a fun project.
Here is the parts manual from Kubota
https://kpad.kubota.com/books/1738
(You may know this next part already but if not maybe it will help)
The three holes on the select arm are where a bearing ball engages giving you the three detent positions. This ball is held into the case by a bolt and spring. The ball bearing is removable. The bolt is located on the left hand side of the case just behind where the select arm goes through the case (see schematic picture). I would check to see if the ball is still in its proper place. during reassembly my select arm got tweaked and allowed the ball to fall into the case.
Also, if the leak is coming from the axle seals they are removable without pulling the case out of the tractor. should be able to slide the seal back and remove a snap ring and axle should come out.
Hopefully this is helpful.