In addition to the lighting situation, which can be paraphrased to “you need only a brake light, operated by either the front or rear control”, you do need a horn and a speedo; or at least some visible means of indicating your speed. This can be a simple (and cheap) digital cycle computer. Bearing in mind the fact that, if you don’t have a speedo drive, you’ll need some form of spacer/cover to go in its place, and that very inexpensive pattern options are available for late speedos, then maybe sticking with a mechanical one would be the way to go. Obviously, a lot depends on your personal preference on aesthetics, and what you feel would look best. I’d probably go for a D shaped Smiths speedo on a custom bracket, because IMHO they look ok mounted that way, and the later types only suit mounting in a cowling where they’re largely hidden.