As far as I know, to restore those old yellowish plastic or rubber, there are some experiments on the web.
They are using Hydrogen peroxide (H2O2) [If I'm not mistaken, correct me], where can buy from those Chemistry Suppliers.

They soap the plastic or rubber in the closed container for 2-3days and exposed by sunlight(or not?), the plastic / rubber will restore its white or even colors.

Search the web and see.