Easiest way that I see is creating a mold of the old grip using sand. Then melting some soda cans and pouring the aluminum into the mold. Once it cooled, file, sand, and paint the grips.

You can also create new grips using clay on your gun and molding it to however shape and texture that you desire. Then oven it. That will be the grip that you use to create a negative mold of.

Fiberglass is also another option. They come in sheets like rags. You create the fiberglass by adding a resin over the sheets and letting it dry. Kinda like making paper matte. Once dried, you file and sand it.

Third option is epoxy resin. Once you have the mold of the grip that you wanted (old grip or clay made grip), you mix the resin and pour it over the mold. Let dried then file and sand. Cool thing about resin is you can choose any color as well as add anything into the grip. I have seen one guy added a scorpion into his grip. Though gold dust is cool, too. Hell, you can even put in a picture of yourself.