Well, I don't think you can actually "design" the greatest go-kart ever. The greatest go-kart ever would be some amalgomation of art and availability and time and place and experience and story and knowlwge increased over all that.
I've never built a go-kart from scratch. I like to buy junkers from swap meets and such and fix them up. Not restore, just fix up and leave a lot of the badges of courage still on that gives them character. Each weld bead on top of another weld bead tells a different story. What kid of 20 years ago has what story that went with this weld bead?
The greatest go-kart just sort of happens. Maybe it's not until you add that 50th story to the 20th weld bead that it just happens to you to become the greatest go-kart ever.