A custom made pack of cells from drill batteries at what ever capacity you requre, or a pack of super capacitors but it may require jump/push start would be my guess but some engineering would need to be done.
Iíve used 4-cell EVO2 battery on my race-bike for past 3-yrs without any problems. Also one on my CBR600RR for about 2-yrs. Saves about 10-lbs off bike.
However, be careful on street bike to never leave anything ON like lights without engine running. That can run down small battery quickly and lithium batteries can get destroyed by single full-discharge. This model does not have over-discharge protection that disconnects battery if charge drops too low. That would mean extra circuits and extra weight right?