Much as I like my Saito and OS 4-strokes, a Kaos is not that kind of plane. Almost any .40-.46 will fly it like it was meant to be flown. I've had one with a piped .46SF and one with a piped .55AX. The .55AX was wretched excess, but lots of fun! Your biggest concern, however, is not power. It is the lousy covering that will be falling off by the first flight. Do yourself a favor and strip the garbage factory covering and recover with Ultracote or Monokote. I prefer Ultracote, but either will be fine. Once you've done that you will have one of the finest flying sport models ever designed.