Well mine did that too, looked like a 45 caliber bullet was shot through my pull starter. Only mine didn't sheer, it just spun off since the shaft moves clockwise.
Now I'm trying to understand did the shaft come off too? Like the nut and the crank arm portion it was screwed onto all sheered off together? If so then I hate to put it too you but you need to replace the entire crank. I mean it's a small small possibility that you could carefully drill a hole in the end, then tap it with threads, and put a fair sized washer and a screw in that new threaded hole.
In my case my crank arm was bent and twisted. So that was the end end of that crank. I got home and just moved it with a pair of pliers and it fell right off.
If I were you, I'd count my blessings that the motor is fine, and I'd try tapping threads and using that to keep it secure
