I'm going to build a battery pack for my TwinStar from C cell Ni-MH batteries. They don't require a stupid charging process, so my alternator won't destroy them. I had briefly considered AAA or AA's, but it would take too many.
I was half joking about the AAA thing, but I use them in my remotes around the house, from cordless phones and the like. The two batteries from your scooter would be great for what you want, if not a bit heavy.
You'll probably want to look into lightening the bike as much as you can before adding weight, and being minimalist in what you do add to it.
That means the handlebar dingy bell has to go, man.
(Probably those grip tassels, too... )