I have two CG Ultimates. One with a OS 120 Four Stroke, and one with a Webra 120 two stroke and cline fuel regulator on it. I have not flown the one with the Webra, since it is not quite finished yet. The one with the OS has been flown many times and it performs all basic aerobatics very well. However it is not full 3D capable due to it's lack of power. If I would use another four stroke, I definitely go with a YS 140 L or Saito 180 and two stroke well as you can see I used the Webra ! If you are not intending to do any 3D flying with it, then I guess your Saito 120 will fly it around OK.
Why don't you consider putting the servos in the tail instead of adding any lead ?
Be also sure to beef up the wing. I sheeted mine because I like to throw it around in the air.
This must be the most easiest and fun bipe out there. You will have a blast.
Just my $0.02