I also have the OS 120 AX engine in my Carl Goldberg Ultimate (From kit) it swings a 16 x 8 APC prop by now at nearly 9,000 RPM, the engine has only 5 flights with my ultimate and I can't describe the joy of this combo.
A good friend of mine has the Venus II with a Saito 1.50 but I believe that he is using a smaller prop. It flies great and have great power to climb, he is an advanced pattern competitor and the only down side is that the stock tank can give him only enough time for the advanced routine.
I am using a 16 oz tank in my Ultimate and it gives me more than the necessary time to accomplish the sportsman routine, maybe twice, I do not use this bipe as a pattern plane option but just want to give an idea of the fuel-time consuming.
You will love that plane; it is on my list of purchases, I am glad that I already have the engine.
Congratulations.