For cleaning, I've done ok with damp cotton buds - just be careful around any stickers and use water to avoid taking off any paint. If they're easy to disassemble, you'll be able to get into crevices more easily.

Stickers can be replaced with reprolabels. White plastic that has yellowed can be fixed with a soak in some dilute hydrogen peroxide with sun exposure but I don't believe that works for colours. There's probably not much you can do short of repainting them.

You can limit further deterioration by limiting exposure to sunlight, and dim/dramatic lighting might help to hide the discolouration.