I used a Saito 50 4 stroke and was happy. Donny says he would use a 65 for more vertical oomph.
I reinforced the gear mounts in the wings with ply gussets and epoxy.
Would recomend setting up wings with 2 aileron servos about 1/2 way out on the ailerons with wiring extensions. No flutter and you can dial in flaperons that make landings a total hoot.