I suspect yellowing is not a surface problem, but right through the plastic. Still, attempting a restoration will be much cheaper than replacing, so an attempt is still worth making.
For hard plastic polishing generally, I use Brasso. Will restore scratched plastic side windows and plastic rear windows in hoods on open cars and that is the usage the vintage car fraternity often use it for. I see no reason why a headlight should not work too.
Car polish will work, but will take longer as it's not so aggressive a polishing agent.
A Dremel or similar hand held wizzy thing will make the process a bit faster.