Was thinking last night about mounting the engine transversely and running a chain / sprocket as suggested and 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 I'm just assuming this due to the diff being on the nearside of the bike)? If so, then it's going to drive the rear wheel backwards.
The only solution I can come up with would be to run a 1:1 Quaife-type reverse box...