is there a reason you need the DC power? i guess lets start there. if you can ditch the battery and the whole DC side of things it makes stuff a lot easier. do you have a headlight in mind?
most of my mopeds are setup very simple, AC halogen headlight with a 1 wire voltage regulator, a LED tail/brake light bulb.
this thread should finally get me to get off my ass and draw up some of the wiring schematics i use commonly on moped /motorcycle systems, been meaning to do that for awhile. Maybe tonight if i have some time when the kids go to bed.