If you used the Reliant engine, you could then use the original propshaft, shortened where necessary. I seem to remember that you could remove the crown wheel and pinion and use a bike sprocket, literally bolted onto the Reliant axle after a bit of light machining. To tension the chain you would then have an adjustable rear axle or a jockey sprocket/tensioner assembly. I personally think that because of probable alignment problems with bike engine sprocket and rear axle sprocket, I would try and fabricate some kind of intermediate shaft which could be tensioned and also re-geared with different sprockets.
