I powered mine with the OS 1.20 AX. Even though it's a larger engine, it fits just fine under the cowl. I did reinforce the firewall at the time of construction (nearly two years ago) and have had hundreds of trouble free flights. It is no understatement how good this plane flies. Two other pilots at my field have also powered their Super Skybolts with the OS 1.20 AX with complete satisfaction. The second photo shows the before and after of how I increased the cowl opening for better engine cooling