Harald:

In order:

If you use a stock muffler something will stick put using either the J-3 engine cowl or the Super Cub. Personally, I think the Suer Cub looks better, never liked the engine sticking out. If you invert the engine completely and use a different muffler, you can probably have the engine fully enclosed with either cowling. On my last one tne engine was inverted with a "Tongue" muffler, only the flat exhaust stack stuck out.

Scale looking bungees aren't hard, there are some good illustrations with the Sig kit you have. Working bungees are a different matter. Do a search, there's a lot of information here already.

As drawn, the aileron servo is right in the middle of the "Sun roof" area. If you use dual aileron servos, or move the single servo to a different location there's no reason not to use a clear canopy section between the front and rear spars in the center of the wing.

Bill.