You could probably use one of these.
Busch + Müller USB-Werk AC
Busch & Müller USB-WERK Charger
(cheaper if ordered from Germany).
See manual link.
Input: 0 to 70 VAC, 0 to 50 VDC
Output: regulated 5VDC, 5W
Apparently it includes a buffer battery, so you may need to add switches.
You'll have to look at the bike's wiring, but I would think there would be a place to get to switched power from the bike (before the throttle).