its yer volts!
check the output of the mag / generator thingy.
Here is the service sheet. http://www.mistgreen.com/s_sheets/SS810.pdf.
Its the second sheet, 810-A.
I wouldn't bother, the lights on all the Bantams I rode as a youff were so crap we usually taped on flash lights front and back. We had more understanding law enforcement officers in those days.