this is real KY redneck but:
might try this?????????
jack rear wheels of tractor off the ground.
put trans in 1st gear
while a buddy sits on the tractor and pushes down on the clutch, as he would if he were trying to shift the gears.
you try to turn the rear wheels forward, (may have to have another buddy turn the other rear wheel forward), so both rear wheels are turning together as tractor was on the ground.
goal: to see if there is any pressure or drag on the clutch system (is the motor trying to turn) As the clutch is fully depressed.
If there is drag, this would mean the clutch plate is not fully releasing, a misadjusted clutch plate can cause the problems you have.
if the clutch is fully releasing, then i would say your problem is in the shifting rods on top of the transmission, they are worn and hanging.
who knows: this procedure might be worth a try.
I am a certified KY redneck, so I have redneck ways, we just march to a different drummer.
Sometimes, even a blind hog finds an accorn.
Note to file: can you turn motor off, then shift to the gear you want, then restart tractor. This procedure would be a pain in the #$(*& but it might work until you can get the tractor fixed.
good luck and let us know what you find wrong with the old tractor.