Cool sounding project.
I'd probably start by mapping out where you will have areas of recessed damage (where you would actually have to cut away parts) with a pencil. Then I'd warm the area (I'm assuming this is hollow vinyl?) either with a hair dryer or warm water and cut away with an exacto knife. Be sure to keep the eye as I'd reuse that piece.
Next step (again assuming a hollow head) is I'd fill the head with something light, but solid like a Ball of tin-foil so you can have a recessed surface to build onto. Get some magic-sculpt (or a similiar 2-part air drying compound) and sculpt the details on; I wouldn't use sculpey since I'd be worried about too much heat damaging the hair. Once your sculpted gore is dried and blended in with the rest of the face and go to town on painting him.