I'm building a chipmunk right now but never flew one, reviews say its one of the best to fly.
It will fly with 55AX but if you are already going through the effort of building one you dont want it not even by smallest chance to suffer from under power.
people who flew it say the OS 61 FX or OS 91 FS 4 stroke ares ideal, I'm gonna use tower 75 for it.
the thing is that if you build it for 55ax you will not be able to change it later that easy.

yeah I need to finish mine, it got stuck to long in almost ready for sanding for covering stage.