Those servos will work fine. As for battery size, well, that's another one of those topics some people argue with a religious fervor. Personally, I don't see the point of carrying a whole weekend's worth of capacity on every single 10-min flight. I prefer to use smaller packs and top off between flights with a $45 peak field charger. On sedate sport planes like the 1/4 Cub, I usually use 600 to 850mAh, 5-cell for RX and 4-cell for ignition. Fully charged, they will easily last 5 flights or more. That gives me a lot of safety margin, when I peak charge every 1-2 flights.

Servo actuated choke is nice, but the carb is going to stick way out of the J-3 Cub cowl any way you mount the engine, so finger choking will not be a problem.

The Robart gear is nice. The WM 1/4 Cubs come with a fairly scale looking main gear already, though not quite as detailed and pretty as the Robart.