Rebiulding a hydro pump or motor is Very difficult and delicate. If you can find the service manual you could rebuild it yourself, but I would highly recomemnd taking it to a dealer, or to a hydraulic repair shop.
I would look for a place that does hydraulic rebuilding. We had an 86 rebuilt 2 years ago by an outfit around St.louis Mo. They told me they would rebuild mine or exchange it for a rebuilt off the shelf. Said my rebuild could cost less than exchange but wouldn't exceed the exchange price. As expected it came close to exchange price. The guy showed me the rebuilding process and there isn't anyway the average shop could do as through of job. They had some plate surface out of the motors on this flat dressing wheel that was putting a new finish on the face. Maybe something like a head dresser although I don't really know how they do that. I guess if you put in all new parts you could probably get it done yourself but don't know whether you would save anything.
Finally got to talk to my brother who is in the hydraulic business. Told me basically the same thing, don't mess with them your self but know a few good shops that do work with them. He also said that most dealerships send them out to specialty places anyway. Thanks for the comments.