ORIGINAL: Charlie P.
IMHO you will need to do better than 2mm balsa to anchor a 30cc engine to. I'd recommend 1/8'' light ply cheeks as a minimum for the front 4'' or so of the model (behind & up to the firewall. I would then drill holes through the cheeks/balsa into the firewall and epoxy toothpicks or (better) bamboo skewers in as reinforcement fasteners.
That's way too much weight and vibration for 2mm sheet balsa. One hard landing and the engine will flop off after it tears the balsa.
Listen to Charlie....thats exactly what I do on gassers. I've used bamboo skewers as well as toothpicks too. Thanks for the pics Charlie!