Hi Steve,
I can relate some personal experience with the RM Jr. When I started back into CL after 25+ uears off, I bought the RM Jr kit and an OS FP20. In the meantime, though, engine output had gone through the roof. So the 20 was way too much power and waayyy too much weight. I had a real pig on my hands. A fast pig, but a pig, nonetheless. The smallest loop it would do was about a foot larger than the lines were able to keep out of the ground. The RM Jr was designed around something like the McCoy 19 RH or Fox 15X. An OS FP 10 puts out about as much power now.

The FP20 can fly an ARF Flite Streak or Skyray 35 through the stunt pattern and not even break a sweat. These are quite a bit larger than the RM.

Anyway, the thing to do is keep it light. Replace any really heavy parts in the kit with lighter contest grade balsa. (The Estes/Sterling kits are not famous for their wood quality.) Keep the finish light, too. Use an OS LA 15 or Magnum 15 with a tongue mufler and start with an 8-4 prop, maybe an APC. One and a half or two ounce uniflow tank and 50-52 foot .012 lines and have a ball.

BTW, the FP series has been replaced by the LAs, and the 20 size has disappeared. If you want to go to a 25, get an LA 25 and a Flite Streak or Skyray, something in the 350-400 square inch area. SIG is working on an ARF Primary Force that will fly great on trhe 25, I expect.

I really enjoy the medium size planes for just futzing around.
Kelvin