In regards to the 6V batt idea. I had one of these set-ups on a motorized bicycle I sold. Worked great for my Headlight & Taillight never had a horn but I'm sure it would have worked.
HT 6v Charging System w/- ground - MotoredBikes.com: Motorized Bicycle Forum
Like I said it worked great for the time I had it. It only cost me around $25.00 for all the parts. (Not including the lights).