I don't know what tools a taxidermist uses on fish casts such as this, but I found a drywall saw effective for the edges of the fins and for cutting out the mouth. I also used a small hand saw, a metal file, a box cutter and sand paper. Below you can see the scooped out mouth and a sample of the cut out material. The whitish edge is plaster and fibreglass. The fish is not hollow, but filled with the yellowish substance that is some type of foam.
So finally, the fish is sanded. Next step? To attach the fins, and drill the holes for the stand. Then hurrah, the project actually begins!