I use Dow "Scrubbing Bubbles" in the spray can to clean plastics. I spray the top of the plastics and let the foam work, wipe it down and use the same wet paper towel to wipe the back. Cleans and shines, and I never had any paint loss. It doesn't do anything for yellowing though.
I broke the only mounting "tip" off of the mountain under the waterfall ramp on WH20. I ordered new ones from Marco then thought, what the heck, and used a drop of Gorilla Glue and stuck the tip back on. It didn't work at first, but once the two pieces got sticky (glue melting the plastic? or just regular bonding time), it held fast and you don't even notice the spot. I have an Op-Pop-Pop with a plastic that is taped together, and plan on trying Gorilla Glue on it to see if it works. If it doesn't I will be interested to see what you come up with for repairing broken playfield plastics.