When you say you can't make it up the hill, do you mean the engine is bogging down? Or do does the engine stay at PTO speed (2600 RPM) but your ground speed slows down? I looked at your old posts and you've had issues with the transmission and at one point mentioned not having a hydraulic oil strainer installed when you purchased it.
If engine RPMs are not falling, it's possible the hydrostatic transmission is passing high pressure oil between the plates and slowing down ground speed. You can fix that without any parts but need to split the tractor to do it. How many hours on your tractor and where are you located