Hi again- I would think that the OS 61FX will fly ur kit built one easily, as I only had to put about 1/2-3/4 throttle to get mine off the ground. I fly mine with only 1/3 throttle, and it cruises around nicely, and at a nice airspeed to do rolls, loops, knife edge etc. At full power, it is a screamer, and a little too fast for me. I did pick up a 12/10 APC pattern prop that I'm going to install for our 3D Test pilot to try out. I think u should give it a try with the motor u have, and make a judgement call after a maiden flight. Let me put it this way, I have a scale Piper Vagabond, just under 11 lbs, and I have an HB 61PDP motor in it, and is more than enough power to fly it. I think the OS 61FX motor has a little more HP than my HB 61, so I wouldn't rule the 61FX out. I think u will be surprised how potent the FX motor is. If, after a test flight, and u think u would want a little more power, u could move up to the OS75AX, which is really powerful, and has the same case size I think.
If ur going to uncover it, look at the structural integrity of the build befor u decide to do some cutting away for lightness. There might be a few places that can be shaved, but remenber, the Chinese ARF uses a lot more inferior wood and ply than the original American GP kit. Let us know what u find, and try to get an appx weight of just the airframe, then weigh the added parts-- motor w/ muffler, fuel tank, battery, servos etc, and then u will know an appx AUW. If ur under 10 lbs, fly it with the 61FX. Dean