There have been a few other options that have been used and I think there are even more that have not been tried. I am thinking seriously of the Polini Thor 303DH. I also think the new engine Badlands (THUMP-AIR 4) is selling would be a great fit. There have been some great strides made with single cylinder light engines over the last few years spurred on by the Paramotor industry. Other engines that have been used are Kawasaki, Hirth, Rotax and I am sure more.