You might want to try either the K&B 48 (if you can find one) or the OS 46 VXDF with the turbax 1 fan unit. More power doesn't always translate into better flight characteristics. With the .45 class of engine in such a light plane you will get nice long flight times and the lighter the plane is the better it will fly in the thin air. Also, check out the Weston UK mini tuned pipe if you get teh OS engine. This pipe is good for another 200-300 rpm.