Seems like the thing to do would just be to kitbash the J-3 to a PA-18 (Super Cub). For that, all you'd need would be to convert the trailing edge from the inboard end of the ailerons to the wing root into a simple flap, and add a wider cowl. There are other differences, but for sport scale, those would do. The simple flap would be much easier to construct and make a linkage for. FX .40 would give you okay scale flight with full span, but I'd recommend something in the .50-.60 range if you want to clip the wings and have any aerobatic performance. If you're really wanting to just dink around and have something with real STOL, shorten the ailerons to about a third of their normal length (just use the outboard third, and make full span flaps from there to the wing root.