I think you have to assume that the tractor needs another engine. Even if the seller is being honest that it ran when parked you have to wonder why it is seized now. If excessive moisture got into a cylinder the rings could be stuck in the piston even if you can free it up. That would require a rebuild (potentially expensive) or another engine. You really can't tell until you get the engine running or tear it down what you are going to run into. If you can do your own work that helps, but you could still be looking at some bucks to get it running right.
Good luck.
JN
Good luck.
JN