How good of an R/C pilot are you? Can you fly the plane on the wing, or do you always run your planes at full throttle?

The low end engine in the recommended range will give realistic (scale) performance. However... the full scale J3 originally came with an engine that required 90% power to get it airborne.

The typical R/C arcraft can take off at under 1/2 throttle (often under 1/4...). Many people never do learn to fly the aircraft... they just guide the engine around. If you are part of that group you won't be happy using less than a .61 on that model.

(I'll bet you get a bunch of people saying to stick a .91 4-stroke in it... when a .50-.60 4-stroke would do fine for realistic performance.)