New guy here, lookin for some help with a broken motor

So my uncle had an old Craftsmen GT5000 that he melted the oil plug on and drained it and continues to run it until she stopped. He gave it to use because we could use the deck. Now that the deck is off, I want to work on repairing the engine as a summer project. The engine turns over too easy. Can someone explain what happened and what is going on inside the engine? And where should I start to begin repairing this thing? Thanks Yall