You did not say what you were doing regarding the rear axle, but if you were going with a live axle could you use a separate big sprocket on the axle and use an electric motor with a small sprocket with a one-way clutch to give a good gear ratio? If the battery or batteries are used just to start your bike engine this may be enough battery for occasional reverse.
https://www.gmnbt.com/one-way-sprag-clutch/