Was thinking last night about mounting the engine transversely and running a chain / sprocket as suggested, and the I hit upon the following problem...
Am I right in thinking that the Rocket engine's output turns clockwise (I've not turned the engine over, so am just assuming this due to the diff being on the nearside of the bike)? And if so, then it's a chain / sprocket set-up is going to drive the rear wheel backwards?
The only solution I can come up for this would be to run a 1:1 Quaife-type reverse box...