I built two of these kits when they first came out. The first had an OS.15FP and it flew well, but didn't have enough power. The second had an old OS.25SF and it was just about right. They both needed a LOT of nose weight to balance them, so your FX would actually help in that department.

My advice would be to install the OS.25FX with a 9x4 prop, reflex both ailerons up 3/32" (reduces tip-stall) and have fun with it.

HTH,
D