Pete, I wouldn't like to try and paint a Ninja. Once the paint started to scratch it would look terrible. If your casing is scratched to bits one way of improving it is to GENTLY go over the plastic with a blow torch. I mean gently, moving the torch quickly over the surface. This can remove any minor imperfections and loose strands of plastic. Not for the faint hearted and no responsibilty if you set fire to your machine. Failing that contact a car bumper restoration company.