Well, everything depends on how much you want to spend. As you plan on taking the engine apart inventory what you need to replace and upgrade. There are all of the engine parts here on the parts site for the z120 engine. You will not be able to replace with original piston size only upgade to the overbore (more power), unless you can find nos piston and sleeve somewhere, and they are out there.
Also, plenty of help on this site and a great little tractor. I have three TO 30's right now from finished overhauled worker to a complete rebuild waiting to happen.