A melted piston.

 

The engine didn't throw a rod, it melted a piston.

 

I suspect it was running rich before it died but I have to confirm this with the owner.

 Another possibility is that it had run out of oil but I would expect both pistons to be bad if that happened.

 

I've found used cylinders, used heads and possibly new pistons if the interchange numbers work but I haven't been able to locate connecting rods and pins. I'm going to try the local junkyard tomorrow and see if I can come up with something.