Steve, I have an Enya .09 in mine. Additionally, I built it with the full span shown on the plans. Note that the instructions show two individual wings with a gap between them, I just filled-in the gap.
I would suggest nothing larger than a .15. Note in the text that they installed a Fox .25 schnuerle. I assume they intended to support some kind of racing event. With that heavy engine, I would guess it would only go round and round.

George