I was a supplier of aluminum castings for Herman Miller furniture and lots of what we supplied was "color buffed" (polished) and clear coated.
We used a clear urethane that our paint supplier applied. It would be similar to what your son's motorcycle parts have on them. I would call around to industrial painters and ask if they can do it.
This may be a good time to consider chrome plating. You would have to find someone who can copper flash and polish first and then lay down the chrome.
Japanese die-cast aluminim is usually an alloy called ADC-12. It's similar to what is used here known as A-380. Both of these alloys have high silicon for fludity in the casting process as well as high copper for strength. The high copper makes these corrode faster and this shows up as pitting and what folks refer to as white rust. Without protection of some sort it won't stop so you are on the right path to seek surface protection.
A call to a local motorcycle shop may also help to see who they use, especially if they do custom work.