I'm far from being an expert at bigger motors (most of my planes weigh about an ounce), but here's a thought... I'm waiting for a AXI brushless rotating can motor for a smaller application, and I'm looking forward to testing their products. You might check the larger ones out (http://www.hobby-lobby.com/brushless_axi.htm). The 4120's can deliver 600 to 900 watts to the prop (according to their tests). They claim performance equal to a 4-stroke 90 sized motor. The large diameter and the way they're built gets rid of needing a GB. These might be a solution...