Airroy, I have a GP GeeBee with an OS 120 and robart struts. It weighs about 12.5 lbs and balances well without extra weight. The 120 flies it well with plenty of speed. You need the robarts because the stock wire gear is too flimsey but of course the robarts add some weight.
I would not recommend the OS 160 because of the extra weight. The wing loading is high enough at 12.5lbs. You will find you need to fly it onto the ground with some throttle. If you try to make an idle approach like you do with a typical sport plane you will be suprised at how fast the bottom will "drop out" as it gets slow. I have a 160, they are great engines, but they weigh something like 8 0z. more than the 120. If you want more power check out the YS maybe.?? DWR.