The parts I'm specifically thinking of are the bezels around the headlights on, say, a Spaceliner tank, or similar. Actually, I have the issue with my Ross Deluxe and Murray Meteorflite. Same deal - the headlight surround is turning yellow or dull grey. I could look for some sort of trick finish, either a candy paint or something that gives an anodized look. I might do that on Ross, since it's customized already, I might just continue down that road. The Murray, though, I kinda wanted to restore. Maybe a patinaed restoration is in order, even though that's a conflict in terms...
I did find a plater in southern California that claims to chrome plate plastic. But they're very expensive. Not sure if its worthwhile, the plating cost would exceed the cost of everything else going into the resto.