Your best bet is to talk to the experts, Mulligans .
I have found them to be very honest and very helpful
They will tell you what units are drop in replacements which they usually sell at the best prices I have seen on the web
They will also sell you rebuild kits
and on a good day they will tell you how to modify or repair your transmission .
Most transmissions have common mounting holes according to their category
Low end ones have the bolts on a smaller base circle and high end ones have them on a larger base circle
You can fit any transmission into any mower it just depends how much fitting you are capable of doing .
The other thing to be weary of is the power drain , hydros gobble up power so you can end up in the situation where the hydro is fine but the engine can not produce enough power to run both the drives and the blades