Sculpey and Super Sculpey are popular sculpting mediums easily available at most hobby/craft stores. It requires heat to cure, so the sculpt time is not limited. It allows for very fine detail work (especially Super Sculpey), and is easily shaped. However, once cured it can be very fragile. Don’t drop your Sculpey sculpts!
Curing Sculpey can be a bit of a challenge. A stand-alone Sculpey piece can be place in an oven at 350 degrees for about 15 minutes, and it will harden as expected. But if you are applying Sculpey directly to a plastic figure, you can’t put it in the oven or the plastic figure will melt. Your options at that point are 1) boil the whole piece for about three minutes, or 2) if the new sculpt is thin enough, you can shoot with a hair dryer for 3 or 4 minutes. I prefer the hair dryer, as there’s less risk of damage to the base figure.
(Just to be safe, when boiling Sculpey I use an old pot that won’t be used to cook food.)