I made the cut. Maybe I'll post a pic later. It worked out really well - I chopped the ears off the X-mount that came with the engine and bolted it to the plane, using it as a template and guide. I used a side cutting bit in my dremel for most of the work, a grinding stone to get it really close, and the best part is that a dremel sanding drum fits right inside the hole, so it came out very round, very precise, and very clean.

I bought regular phillips "cheese head" screws to mount the engine, 12mm long was perfect, and there is just enough clearance on the prop side for the prop adapter.