Probably the best way to do that is to remove the engine mount (if it's there) and screw the screws back in flush with the rear side of firewall. This is done to keep the glue from filling the screw holes if blind nuts were used to hold the engine mount to the firewall. If there were just some drill holes then disregard and proceed further. Now go and get yourself some glass cloth (medium to heavy weight will do) and with a very sharp pair of scissors cut a piece to match the size of the firewall. Spray the backside of the firewall with a light coat of contact cement and let it get to a tacky state and then place your cut piece of glass cloth on it and smooth it out to where no wrinkles. Then mix a good size batch of 30 or 45 minute epoxy up and with the fuselage vertical on your bench (maybe leaning against the wall or something) pour it in onto the glass cloth and "dab" it into the fibers using a cheap brush or even a foam type brush. Don't go to heavy with the epoxy and try not to push the glue around too much to where it'll move the glass cloth. The idea is to get the epoxy to soak through the fibers of the glass cloth and good contact made to the wood of the firewall. The epoxy will flow a little on its own. Some guys will actually add a small amount of alcohol to the epoxy to thin it out which helps and it'll also increase the amount of working time. Be careful when doing this since very little is needed to change the consistency of the epoxy. Use alcohol soak paper towel to clean up the area's outside of the firewall which may have gotten a drop or a splatter of glue. Let the work sit for a few hours and then go back pull the screws out (if you had them screwed in into blind nuts) and drill out the existing holes on the firewall. Should look pretty nice and added a good deal of strength to your firewall.