Most models that I have seen the motor and mount bolt directly to a piece of plywood which is then glued to the fuselage. For slow flying or trainers this will do. But for planes that are going to be under high Gs and do snap rolls this is probably not enough support.
All you need to do to add some extra strength to the firewall, is run some self tapping screws into it. Here is what to do. First before covering the model laminate a piece of 1/32 inch plywood to the side of the fuselage that is just as wide and about 4 to 9 inches long depending on the size of the plane. Cut one for the other side and epoxy them down. Their job is to stop the screw heads from wearing through the balsa or pulling through they also add lots of strength to the nose. So make sure they cover the area where the screws will go into the side of the firewall. The firewall should be glued in place like normal (now would be a good time to finish covering the fuse with whatever you were using i.e. Monokote) When the epoxy has cured and covering is taken care of use a 1/16 inch drill bit to make pilot holes through the side of the 1/32 ply, fuselage, and into the firewall block (probably plywood as well) Make sure you get them straight and in the center of the firewall side. Make two or three holes per side depending on size of the plane. Use self tapping or sheet metal screws to secure the firewall to the fuselage. Before you screw them in apply epoxy to the threads for added support.