If I have to replace a firewall, I usually use 1/4" marine or aircraft plywood. this would have 5 or more ply laminations, rather than the three laminations reqular plywood has. 1/4" ply easily holds engines up to 75-80 cc. You may get away with 3/16" aircraft ply, but these planes usually tend to be tail-heavy, so the stronger, heavier wood couldn't hurt.

Good luck with you repairs.

Mike Boyd