I just started blender and I was trying to make a go-kart myself. Seeing yours really helps a lot.

I will be going back and trying to retopo the seat and other elements. The current model can as is be brought into daz studios and is a good fit to the genesis figure.

Some additional info. A Bezier Curve was used to create the main frame. I then converted this to mesh data. The honeycomb addon was used to create the wire mesh for the raditor. Tip: once scaled to the size you need simply duplicate and use snapping to create new segments and remove doubles. I tested out the model withn unity with the fbx export option. Tip: apply all modifiers and join all of the parts to the main frame. Keeping the wheels as seperate objects. Hopefully this extra info helps you out