You can buy a heli coil repair kit for $20-30. They aren't hard to use and usually have the special tap, an insertion tool, and sometimes even the right drill size. I haven't finished the motor I'm building and I already heli-coiled the cases just for insurance. I also ride a 1971 Yamaha 360 Enduro and I must have a dozen in it! 50 y/o aluminum threads are just about hopeless. Heli coils to the rescue.