In my case at least, the starter apparently somehow caught funny and there was a bang! and that was that. Now I always keep my thumb on the button until after the engine starts and the relay turns the starter off. In 4G's case, maybe his 750 kit was too much for it. They're pressed together, not welded.Bent crank? How does that happen in routine operation?
If it is the crank, the good news is they're fairly cheap on ebay, and occasionally even cheaper here in the classifieds. The rods will be attached and you connect them to your pistons using new wrist pin clips. I've also heard of pitted bottom end bearings making noise