i agree with polarbus. i would not take a chance on cutting a new keyway. sounds like a disaster waiting to happen. if you cant afford a new one now, you would be better off to save up for a while and buy the proper mag. seriosly, i wouldn't try the repair. think about what pre-det. does. it fires the fuel/air charge while the piston is still on the way up tries to force it back down. bad, bad, bad things can happen to your motor. motor repairs could cost more than the proper part. not being a jerk here, just trying to help.
maddoggy