You can try washing in Dawn dish soap, which should take off some of the dark film if it's caused by smoking, But a lot of dolls, especially in the 64-65 time period, have heads that darken, sometimes significantly. It's just a property of the vinyl used in those years and, unfortunately, it's not reversible.

IAgree, the stain is nicotine exposure - just like smokers get on their teeth and finger nails