Seems like the OS .90 FS, may be a heavy engine for this airplane. I had a .91 four stroke on mine for a while, but the extra weight and change in wing loading, I went to an OS .50sx with a Tower muffler. I have used either an APC 12X4 or 12.25X3.75 props. I was using an APC 15X4 on the .91 and had to use quite a bit of aileron to conter the engine torque effects with the bigger, heavier engine.

The .46-.52's weigh around 17 ounces with a muffler, the OS .91 Surpass weighs 24.2 ounces, where the .91 Saito is 19.6. The new Saito .82 is around 16 ounces. Most of us have found the best performance on the two strokes with the Tower muffler. Rapid throttle response and better weight make them most advantageous. The bigger, heavier prop on the front of the .91 also adds weight, compared to the lighter 12 inch props the 2 strokes use.

I am now using a cheap Magnum .52 with a Tower Muffler and hover at about half throttle and can rocket out of hover with that engine combo. Many of my 3D club members are using the OS Hype .50 helo engine. The plane is much lighter, with the lighter engine, so the other 3D stuff goes better as well. Key hint is to watch the nose and keep it in the sweet spot. I have found that once you find the sweet spot and can keep up with the nose, you don't need allot of throw to keep it in a hover, as long as you stay ahead of it. If I was to recommend a .91 four stroke for this plane, it would be a Saito, since it is nearly as light as the .46 and .50's and has plenty of power.

I have found mine to be much easier to sustain a hover with lighter weight and the cg as far back as you can handle it. A nose heavy plane just doesn't want to dance as well as one with the cg back.

Good luck, you got the right plane for the job, now just practice, practice, practice.