RE: 1/3 Scale J3 Cub - Engine Recomendation
It all depends on what it weighs. I have three of these planes which I use as research aircraft and they are nice flying planes.
I flew one with a 3w-75 single cylinder with lots of power and we used to fly it up to 2500 ft agl to take atmospheric readings. Takeoff wt = about 50-55 lbs.
A second one had a 3w-85 twin. A little more power but a lot smoother. The 75 single is rated at 7.5 hp and the 85 twin is rated at 7.8 hp.
If you just wanted to putt-putt around the sky cub-like and your weight remained below 40 lbs, I think a G-62 would be the right amount of power. The kit is rated for a 50 cc gas engine but I think you would feel more comfortable with a little more power.
The current versions that I use are equipped with slotted flaps for slower landings and powered with 3w-106s (the same modeltech planes above which have been altered). Takeoff roll at 65+ lbs is about 20 ft with flaps lowered.
Hope this helps
Elson