Woodruff key is only for locating harmonic balancer, bolt holds balancer on!
Very common after timing belt replacement, Especially RWD vehicles (FWD usually has easy access for rattle gun).
No one removes starter motor to lock crank for correct tension on bolt!
Remove balancer, line up tdc on crank, fit new key(glue if req), line up tdc on balancer, tension bolt to specs!
Have done a few, inc above or full removal of crank, spray weld snout and re cut slot, but no warranty applies either way!
My advise: try it, if it slips again buy a new crank!