I never worked with manganese alloy which I didn't even know that it could rust! But as far as getting touch up paint, all you have to do is take the bike down to a hobby store and try to find a bottle of Testors or some other brand of paint that closely matches. The only other thing you can do which is expensive, is to take the bike to a auto body paint shop and see if they can match it, but they will probably only sell you nothing less then a quart. I would try to match it as closely as I could with Testors or some othe brand of model paint.