You would be better off if you just cut an entire new firewall. If you try to fix the current firewall you're going to find it breaking again if you have another rough landing. The firewall is too important of a piece of a plane to take chances on, if it's not structurally sound you'll find it pulling away from the rest of the fuselage. And we all know that's not good. My advice would be to get some new ply and cut a new firewall. You should be able to remove the old firewall using heat. The heat will usually soften up the epoxy that holds it in and allow you to pull the old firewall out. Then clean up the wood and epoxy the new firewall in place. Don't forget to seal the ply so that fuel and exhaust residue won't soak in to it.

Hope this helps

Ken