I'm not really too familiar with that engine. From the information on the RCV site, it weighs about 5oz more than the Saito 180, so you may need an oz or so of lead in the tail or move the servos aft of their stock position to get your GB to balance. The rudder and/or elevator servos could be hidden under the elevators and mounted on the fuse exterior like the aileron servos are mounted under the wing for easy access. The RCV 120 has a similar hp rating to the OS 120, so power shouldn't be a problem.

I've heard that the RCV 120 needs a different cooling strategy because of its piston arrangement. If you are going to use the dummy radial, you could make a cone to force the air into the cooling fins like has been mounted to the cowling in the pictures in post #6 of this thread. http://www.rcuniverse.com/forum/m_88...tm.htm#8870699 The fan and the other baffling shouldn't be necessary because the cooling exhaust area created by the GB's cowling should be able to make plenty of negative pressure to pull air in through the front. The fan was only needed in the model Spitfire because the air inlet is obstructed somewhat by the spinner.