As to the PTO, if it is attached to an implement (a rotary cutter, for example) is there enough PTO drag to drive the mower at all, or just enough drag to make the bare tractor PTO shaft turn?
On the left side of the tractor there is a valve assembly with ports to check the PTO clutch and brake pressures. They are labeled.
PTO clutch pressure should be "0" with PTO "off", and PTO brake pressure should be 150 PSI.
If that's what you find, the PTO brake is worn out, for sure, and if the PTO turns with any force at all there could be PTO clutch damage, as well.
PTO clutch repair requires a split at the engine, PTO brake repair requires a split between the clutch housing and rear end.
The most LIKELY scenario is that the PTO clutch is fine, and the brake has worn out or failed, connecting it to an implement as I mentioned in the first sentence will give you an idea if it's something you can live with, or something to walk a way from, as you don't want to pay the repair bill on this if it has to be split in two places to get the PTO working to your specifications.