Often the clutch plate gets stuck to the fly wheel and won’t release when you press the clutch pedal down. If it was sitting for a long time without the clutch pedal blocked down I would assume that is the problem but
@Dave_eng post about double clutch plates may be at odds with that diagnosis! Can you start the tractor in gear with the clutch pedal pressed down? If so then your drive clutch plate is not stuck but possibly the pto clutch plate is?
On my older tractor there is two clutch positions off or on (no go or go). That is the same clutch for the PTO. On your tractor it may be push clutch half down and not go but PTO still going, press clutch pedal all the way down and everything stops.
Do you have a a two stage clutch or do you know?
Splitting the tractor will get you access to the clutch plate and pressure plate. I would replace the pressure plate, the clutch plate, the throw out bearing and the pilot bushing in the flywheel if I split the tractor. You may want to even replace the rear main seal while you are there.
Splitting the tractor will get you access to the gear case and PTO GEARS but not the PTO shaft out the rear of the tractor. Taking the top cover off the transmission where the shifters are gets you access to the shift forks and allows for some repair of the shifter mechanism and allows visual access to the gears in the gear case (may be additional covers to remove on top of the transmission)
The front axle will have a center pivot that will allow the engine to flop to the side so you need to put blocking from the axle to the engine before you remove the rear half of the tractor. You will need to support the front half and the rear half of the tractor while you spread them apart so that support needs to have wheels on it.
Kubota USA dot com has great illustrated parts list to see how many of these things go together.