I started out with a 25 2-stroke and it was not good power and I have read many posts about engine size for this one and the 56 would be too heavy for it so I went with a 40 4-stroke and it turned out to be the perfect match and balenced without adding any extra to it, I ran mine with a OS 40FS with a 12x4 prop and it was just right for this little plane and Saito makes a 40 now with a little more power than my old OS So I would try that one.

Hope this helped
Dauntae