The "automobile" as legally defined in the U. S. and U.K. Is going to be a heavy load because of crash & safety regs. I'll point out in England, the 3 wheeled vehicles evolved to avoid automobile taxes and regulations. From the iconic Morgan to basically road going wheel chairs for the many Veterans, there was a wide range of "things" on the road there when I was younger. in the U.S. the cost to make a light weight Mini Cooper that is road legal is way too high. But a scooter with a sidecar is not. The difference between scooter & motorcycle is mostly speed. for stability, the "tadpole" trike arrangement, like a Can Am Spyder https://en.m.wikipedia.org/wiki/BRP_Can-Am_Spyder_Roadster is a good choice. Better/lighter/cheaper is... https://ultimatemotorcycling.com/2018/09/23/2019-can-am-ryker-review-20-fast-facts/ Or you can design your own..... Here is state of the art. Or maybe something slower?