I think it really depends on how you intend on flying the plane. I'm flying one with a Saito 120 right now, but I also had one with an OS FS91. The flight characteristics are completely different. A bigger, heavier, engine will feel more like a hot-rod. A smaller, lighter, engine will be a better all-rounder. In my opinion, the sweetest handling will be found with an engine weighing 24 ounces or less. This will limit your choices quite a bit. If you're also on a tight budget, you really only have two choices. A very strong 60 or a light 90. If I were buying a 90 2C, I would look very carefully at engine weight with muffler and probably choose the lightest one. The MVVS looks good. You may also wish to side mount the engine and use a Bisson muffler. This will save you a couple ounces but cost $40. In all honesty, I think the Saito 100 is a perfect match for this airplane. It will give you everything short of 3D performance. If you're going to do hoverbatics, you'll need more power. A TT120 two stroke would be a monster for little money.

BTW, my current flying style is more IMAC oriented. I use the power of the Saito 120 to draw long vertical lines. I only hit full power 10% of my flight regime. It lands like a brick with parachute deployed. My airplane is 8.75 lb.