Ok... your post doesn't make much sense....
If the engine spins, the piston can't be seized. If it runs, it's definitely not seized.....
Did you overheat it.... well, maybe. If you did, it was because the back or inside of the jug is covered in crap and it can't cool. It is an air cooled engine.
If it stopped suddenly, you could have galded the rod to the crank, or you could have scored the piston.
Will adjusting the valves, checking the ignition, and torquing the head help.... NO.
You can do two things:
*Tear it down and begin the rebuild.
*Take the engine out, remove the tins, clean the outside of the block (If it is in fact dirty), tune it up, maybe put on a head gasket and run it for a while and see what happens... Then you will likely end up back at option one after spending all that time and effort.
Your choice. I'd get ready to rebuild myself. Or start looking for another engine.