A .60 two stroke is a bit much and a .50 four stroke is weak.
I built one according to plans and flew it for years on a variety
of engines. A good .45 two stroke or .65 four stroke seems to
be about right. I had flaps and they are worth it for the fun
take offs. The airplane as designed is very strong but if you
load it up with a bunch of stuff it could use a dihedral brace.
That means six or seven pounds extra. It will,however lift it
with a .60 two stroke it just won't survive the first turn.