Not having any pictures to look at, I would recommend replacing the whole firewall. Since the firewall is the mounting point for the nosegear and engine, as well as a major piece of the main structural frame work, it needs to be as solid as possible. Therefore, JURYRIGGING THE REPAIR IS NOT ADVISABLE. Have someone from the club that is experienced in building and repairing R/C planes help you REPLACE THE FIREWALL, not just patch it. I'd hate to see the firewall fall apart in midair. Need I say more?