It sounds like you burnt the pistons. Probably #3 since it gets the least amount of air. And probably the valve guides too. My suggestion, rebuild the whole engine. Don't bother trying to diagnos the problem. You know what you did to do her in.
Aircooled engines rely not just on the air blowing through the cooling fins to cool it down but also to cool the oil. Oil heats up and breaks down and then you have 300+ degree oil moving through the engine. Chances are the rod bearings went to hell in 2 miles time.
On type1/type2 motors you always want to have a spare belt on hand. Only type3's can run without the belt for ANY length of time or until the battery dies.
Your oil must've been boiling when you got it home.