This happed to me. I broke off a head bolt when I wanted to do a valve job on my non i/c 12.5. It was what had come on the tractor originally. I tried to use an "easy out" to get it out, but that broke too. I got really pissed off and bought a new "used" 12.5 i/c from a shop near me. It was missing a carb and starter. I swapped everything over with all new gaskets and put in an ohv piston and connecting rod. Lots of porting, torquing, never seize and loc-tight... lol
But it's a tough little motor. The new block cost me $10. All the gaskets and other stuff cost another $15-$20.