https://www.motherearthnews.com/gree...r-zm0z13amzmar
It could be a starting spot.
I agree with FreeBeard about using the classic VW bug platform as it gives you a multitude of kit car choices for bodies. You can adapt various engines to fill your needs.
The suggestion of a diesel as your choice for an engine is a good suggestion to follow. The choice of a Kubota 3 cylinder does not give you much in the way of tuning but does give you a high starting point for thermal efficiency without much effort.
But, if I was you? I'd get a manual front wheel drive with a substantial overdrive and stick a Kubota 3 cylinder on it. Then I would cut off the back end of the vehicle and weld a substantial motorcycle frame and drive wheel on there. That back wheel will be electric drive and will provide a boost during acceleration to make your vehicle more enjoyable and safe to drive. Then build out the body in as streamlined a tadpole configuration as you can.
Or just contact R. Q. Riley.
https://rqriley.com/