CG
I have built two CB ultimates with OS 120s (just finishing the second one now) and I just moved the cowl forward about 3/8 in. Frist one had the battery behind the cockpit and the new one has the elevators servos in the tail and battery forward. Don't know what the weight difference is between the engines but I hope the helps. BTW the frist one weighed 9 1/2 lbs, by using carbon fiber gear, light weight tires and other changes, this one will be about 8 3/4 lbs.