When I started the only kart frame style I had in the game was a fairly generic, 1980s racing frame. It took me a few days to model based on a handful of photos I’d collected during my research.
There are many versions of this design in the wild, and especially in other go-kart games, so I’ve tried to keep all the other styles as different as possible.
For the “dirt” frame I wanted a lot of clearance and a tight, squirrelly wheelbase for dodging rocks and trees. This is the boxiest of the frames, and I think it looks like the dependable clunker it is meant to be.
There are 7 frame designs; a few are based off what I saw as common vintage kart styles but I’ve added a couple of my own fantasy kart designs you’ll be able to acquire late in your career. There are literal hundreds of frames and weird parts I’ve seen doing research that I wish I could include but less is better for this size of game. It’s a bit like the chassis’ in Mad Max - they each have their benefits and weird quirks but it will come down to which you think looks nicest.
The last step will be completing the textures for the karts. For now they can be painted in different finishes and I’m working on dynamic scratch, rust, and dirt buildup.
These are placeholder colours and textures. The blue is scratches, red for rust patches, and a top green layer for dirt and grime. Each of these passes affects the specular, roughness, and normal layers of the texture in the shader.
The finished look will be like this but with more appropriate texture masks.
Of course there will be options at the paintshop for a quick wash and paint touch-ups so you can keep your beauty fresh and clean. 
Be well and good luck with your games!