If you cant get new plastics and have to paint them, I would reccomend Krylon Fusion paint in a spray can. You should be able to find it anywhere they sell spray paint. Just prep the plastics as it says on the can, and spray it on. Dont paint the gas tank, as the paint will just end up peeling and looking crappy eventually.
To my knowledge, there is no paint that will adhere well enough to withstand MX. I have used Fusion, and now I want it off because it has chipped, scratched, and rubbed off everywhere. We had a discussion here on this a few months ago.