I have a .72 Saito on my H-9 Cub, and it just loafs. It easily turns a scale diameter prop, and sounds very cool just puttering by at 1/3 throttle. A .56 would fly it fine; but have you seen the new .62 Saito? The .65 is discontinued, and the .62 is a bored out .56. Same size as the .56, a couple grams heavier, but makes more peak power than the .65 did (1 hp vs .95 ) You could hide it completely in the cowl if inverted.