Some plastics react with chemicals can melt with Meths, turpentine etc so it's not worth the risk.
If the yellowing is due to ageing and General decomposition of the plastic due to light and heat, sadly it's just the way it is.
Personally I would just clean best you can and leave it as is, wouldn't put any cleaning/ chemicals. It's not worth the risk, and TBH, once you have the figures sat on there and the backing card in place, it certainly will detract from the yellowing mate.