The drive train(Engine, trans, swing arm and final drive)on my '91 K100RS was corroded when I got it. I hit the loose corrosion with a wire wheel on a drill motor followed by a couple applications of Aluminum Jelly.
After the corrosion was cleaned up, I primed with self etching primer followed by a couple coats of satin black engine enamel. It's been three years now and about 12,000 miles and the castings still look pretty good.