Knife, flathead screwdriver, and elbow grease.

Thanks for the post. No chemicals needed?

No, it's not really necessary. Boiling the skull plate for a little bit will help soften up the remaining bits of tissue. Just keep the antler bases out of the boiling water. A stiff nylon bristle brush for scraping paint also helps remove the little gummy bits clinging to the antler bases.

If you need to replace a lot of material, you're probably much better off building up with wood attached directly to what remains of the skull and shape it to look pleasing. This way there is less chance of screws tearing out. I'm guessing that everything is going to be covered with leather anyway.

I'd request a Van Dykes catalog just to get some ideas. It doesn't cost anything.

http://www.vandykestaxidermy.com/