It's hard to tell how "stuck" the engine is because if it's been inside it may not be all that bad. You can try jacking up the back of the tractor and putting it in gear and rocking one of the rear wheels back and forth to see if the engine will free up.
If not you'll need to take the engine apart to rebuild it. There is a one piece head and two blocks. The head and blocks are easily removed once the side cover has been removed. It is true some shops have a hard time testing and boring those blocks because they have a blind cyl hole. Good luck, a ZA is a great project, parts are still redialy available.