I think this can be challenging to DIY. You can paint plastic with a high copper content paint to provide a base to electroplate onto though. After that, you can plate the product with other metals that stick to the copper base you just put on. Can get expensive quickly and may not be worth it for one part. You might be better off getting it plated locally if you can find someone to do it.

This company sells kits (and the copper paint) if you want to take a peek. I've never used any of it, but found it when researching DIY electroplating myself. So far I've only worked with copper plating.

http://www.caswellplating.com

Another option might be gold spray paints if you're handy with a rattlecan. Here's someone who did a comparison of different brands of gold colored spray paints; there is quite a variance between the brands!

http://www.brittanymakes.com/2013/04/03/finding-the-perfect-gold-spray-paint/