Rick,
Unless you have lights like one of the crazy scooters I saw when visiting the Ace Cafe, #14 AWG (or 2mm, metric) should work fine with our low current demands, as Scotty has suggested. You could go larger, but I would go no smaller. One thing about bigger wire, if you should you have a momentary short circuit, like a slip of the hand, your wire is less likely to melt the insulation or act like a fuse. #14 is probabaly fine in this way.
By the way, if anyone knows, I am curious if there is a clever British nickname for the type of scooter with a million lights or mirrors.
Richard L.