Assuming you want to keep the 6v electrics, you could put in a dc dc step up converter and a 12v outlet.
These are inexpensive via eBay etc. and you could also run goodies like a HID headlight.
See entertaining youtube video... http://youtu.be/p81fld-i-98
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.