First let me say welcome back to the hobby and welcome to RCU!

1) I would plan on a 1/4 scale cub. I would also recommend building a clipped-wing version . A clipped-wing Cub flies just as gentile as a full-wing version, but frankly, the full-size wing is too big. It was designed around an engine that was barely big enough to get the plane off the ground and due to it's length, it makes turning difficult (Lots of adverse yaw).

2) Take the pull-start off. It would be nice to pull-start it, but as you said, it's excessive weight - not to mention the fact that it would be impractical (Where would the handle be sticking out?) and it would be very difficult to conceal under a cowl.

3) Keep your radio at least 12 inches from the engine and use a non-metallic pushrod for throttle.

4) I have not used this particular engine, but I have converted weed wacker engines and they work great.

5) Gassers have a pretty wide range of props. You COULD use a 3-blade, but it wouldn't look very scale, and 3-bladed props lose some efficiency, I would try something like a 16x8 2-blade

6) That muffler should work just fine.