Have you thought about writing to hydro gear? I had a similar problem with a gearbox on a toro mower and wrote to General Transmissions and they were able to send me a diagram and supply me with a part which is unavailable in Australia. There is probably not much difference between the models apart from outside body configuration. What is actually broken. An internal or external part. Single model hydro gears are usually non serviceable and mower repair shops take that a step further and also say they are non repairable which is not usually the case for a home mechanic. As I said, contact Hydro Gear and they should be able to help.
