I was involved in the test flying of the Aquila prototype, using the OS 120AX. There's no reason you could not use a 4 cycle engine. The main thing to keep in mind when selecting an engine for the Aquila is that it is designed for an engine in the weight range of the 120 AX, so consideration of CG with other selections should be done. The performance was good with a muffled 120AX, and very good with a tuned pipe on the 120AX. For perspective, I used it in Masters competition as a backup on a windy day. My Prestige had snapped a rudder cable and I had all of about 3-4 flights on the Aquila. It kept me in 3rd place and it wasn't trimmed yet. This was with a muffler, and it had enough to perform in 15mph cross winds. With the extra power on a pipe, it handles wind and the Masters sequence with ease. This is another way of saying that you don't even need a 1.40 size engine to get performance from this design.