A standard Woodruff key should be OK, but I've never seen a worn key that didn't get that way because of a loose-fitting balancer. The balancer should be a TIGHT press fit on the crankshaft snout- - - -either installed with a BFH, a jackscrew pusher, or by heating the balancer hub until a drop of water sizzles on it, then tapped on with a block of wood and a hammer.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!