What problem do you have with the hydro?
The 50 series had a well made hydro (Daikin, I think) but it is now approaching 30 years old so it may me time for a set of valve plates.
Before you gut it, I would suggest you have the local dealer or someone who know what they are doing to run some pressure tests on the charge pump and the drive pressure. It could be a charge regulator issue or a high pressure relief valve if either pressures are found to be low.
Good luck!
YC