You certainly won't need much, or any, extra nose weight to balance it,(using the .90 twin) and if you had to add tail weight, the model can handle a good bit of weight gain and still fly great. What the others said about .50-.60 size 4 strokes goes for me, too. I'm a Saito fan and I have a .65 in mine, mounted inverted. The valve covers just barely contacted the cowl bottom and I have small cutouts to clear them, hardly noticeable. The newer .62 Saito is a shorter engine, basically a bored-out .56, and it will fit completely in the cowl. I found the two scale cowl inlets to be plenty, and just made a slot at the lower rear of the cowl for a cooling outlet and have never had problems with overheating. Although the .56 is plenty of power for scale flight, a .60-ish engine allows use of a scale diameter prop (14 in.) Anything bigger is a waste of motor, IMO, it really won't fly much faster, and has plenty of wing to be used for a towplane without needing a big engine.