drill the screw out one size smaller than the threds and chase it with the correct size tap.it alluminum.it shoulnt be a problem.worst case drill the diameter of the busted screw 0ut and fill the hole with jb weld and retap to the correct size.if you have accses to a lathe you can make it perfect.i did one on the end of a crank and it gave me no problems. i did that by hand with a drill. use a little macyverism,you will get it.

darren