Can you verify that the bolt that held the lower pulley and clutch on, fed right through the extension and screwed into the end of the crankshaft please?
The engine problem looks much more like lubrication failure than overheating. Overheating, in my experience anyway, leads to ring/piston seizure, not big end seizure. My guess is that there was very little oil in the sump and he was running it hard. I understand all of the OHV v-twins have pressure lubrication, so if the oil level was not low, there may have been a failure of the pump or filter, or a blockage of the pump intake in the sump.
The crankshaft would have to be ground if you wanted to use it again, and you'd need undersized connecting rods to match. I don't know how practical that is, but it is a modern engine so it may be easy. A short block (short motor) may be a more economical solution, depending on the other damage inside the crankcase. The hole in the crankcase looks easily fixable by an argon-arc expert, if you wanted to repair the old engine.
