Alan, it is my intention to try this. A Traxxas Rustler or TMax clutch bell meshes with the main rotor gear perfectly. The gear pitch is the same. The problem is the stock crutch. As it is there isn't room to mount a .10 or .12 size engine. I have an O.S. CV12 engine that runs but needs to be rebuilt. I have started an aluminum crutch with enough room to mount the engine. There are a lot of issues but it is possible to do it. Right now though, I'm enjoying flying too much. This is my rainy, snowy and windy day project so it will be a while. A cv12 produces .56hp and is rated at 30,000 rpm. It weighs much more than a vmax7. The rpms could be brought under contol any number of ways. The center of gravity would have to be fixed. The larger heat sink would be an advantage I would think, but O.S. engines run cool anyway. If you wanted to skip the crutch part you could look at "the man from moon" crutch. He's already mounted a .15 size airplane engine on it but it ran hot (see what I mean about the large heat sinks of r.c. car engines). I predict my trial runs will be sometime late winter or early spring. My r.c. start was in 1/10 scale racing and I still race today (Losi rules!). But I just love the helis. It's the most challenging/rewarding/fun hobby I've ever tried.
Mark