OK - Now I may be wrong, but I would at least try to use the JB Weld - with the part off I'd see if I could get the JB Weld into the crack and a little on the surface, but not inside the center. If there is enough surface to do it - I'd use a hose clamp to hold it together while curing (hardening). After it had cured I'd make sure none of the Weld material was inside or in the keyway slot and try it - If it holds Great, if not- you'll need a new one and all you lost was the cost of the JB Weld and a couple days.
Unless you hit something or use a pull start cord that engages this part - there isn't usually a great deal of pressure on this part - JB Weld is amazing stuff.
It depends on your finances and how much the new part will cost.
Maybe I'm just remembering when I didn't have ANY extra $ for anything and would try to fix everything if it could be fixed. :biggrin:
Good Luck !
What did you hit to shear the key ?