Interesting,just 2 days ago I decided to throw out an orphan rear ESGE just like that (35 years was just too much to save it any longer). The front was destroyed when I collected a stick between the fender, tire, and fork. The yellowing of the plastic is likely to be not just the surface. I've never been able to remove it with simple polishing. If anything works, it will be wet sanding with sequentially finer grit followed by compounding. It will take many, many hours to achieve satisfaction (if possible). I think these were a thin sheet of Al sandwiched by plastic.

I will retrieve the tossed fender and dissect to provide a better opinion. Stay tuned.