what about just lining it up and going through the factory box just leave the factory box in 4th that way when you stop you can still use reverse.
. connect the bike clutch to your clutch pedal etc
basically you would have 2 gear boxes but im sure making a coupling to go onto where the drive sprocket for the bike and connect that into the gearbox would be a lot easier than using electric motors and stuff .
also it would give you a whole lot more ratios as you could use all the other gears in the origal box , bit like low in a 4x4 gear box