IMHOyou will need to do better than 2mm balsa to anchor the firewall ofa 30cc engine to. I'd recommend 1/8" light ply cheeks as a minimumglued to the balsa sheeting for the front 4" or so of the model (behind &up to the firewall). Plus tri-stock inside. I would then drill holes through the cheeks/balsa into the firewall and epoxy toothpicks or (better) bamboo skewers in as reinforcement fasteners. You can see the ends of the dowels in this image. The plugs still sticking out are to keep the engine bolts from crushing the firewall.

That's way too much weight and vibration for 2mm sheet balsa. Onehard landing and the engine will flop off after it tears the balsa.