Half of that is still 8000 pounds worth of force. Lots. Esp, on a smaller tractor. 16000 pounds is a lot of force! If you don't have some background in cable safety, it'd be a good thing to read up on. A snapped cable can kill ya pretty messy.
If you are able, bend up a crank out of 1/4 inch rod to be able to spin the input shaft with, nothing too fancy, just something to stick in the bolt hole. If I am right, the lever is a dog clutch to allow you to pull or lower under load, and the knob is the freewheel unit.
Not really rocket surgery, but finding parts to replace anything broken inside the gearbox is probably a non-starter, and it might be worth a poke and prod there, instead of after it goes home with ya.
It'll give you an idea of the ratio of the gearbox, that at 540 rpm PTO speed will give you some idea of how fast it may be. Or how slow.
