I never did shifter karts, but I raced this Yamaha with a modified 100cc motor on dirt for a couple years in the early '80's in South Carlina. I sold it and bought a Margay (if I remember correctly) and put a 100cc rotary valve motor and ran it on dirt and in sprint races on asphalt.
You are correct in that it is a lot of fun. I quit when others started showing up in 30' trailers with spare frames, motors, etc., and a 16 year old driver flew in on a helicopter. I was there for fun. I went to flying radio controlled helicopters.
Good luck with your racing and stay safe.