Hi again,
See the reply under Engines for Airplanes abou FPs and LAs. But I do want to add a note about the Ring Jr. It is a pretty small ship-more in the class with the Sig Akromaster. So it's more of a 10-15 size plane than a 20 size. The difference is that engines have gotten much much powerful for their size since the Jr was designed. A really light RM Jr would probably fly on a Norvel 074, for that matter.
But to get to the point, the RM Jr is fun to fly, but limited by a thin airfoil and not much wing area. It will need to be kept light to fly well. So don't hesitate to replace hard kit wood with better contest grade stock. There aren't many parts, so it shouldn't take much time or balsa to do it, and it will make a big difference in how it flies.
A big step up would be a Sig Skyray or Flite Streak (ARF or Brodak kit) with an OS LA 25, or the FP 20 if you have one. These will do the entire stunt pattern in good style, where the RM might struggle.
Just a thought for future reference. All the best,
Kelvin