Mar-Hyde Tal-Strip II Aircraft Coating Remover is the best stuff you’ll find for stripping paint from a kart frame if you can find it. I used to buy it from Kragen Auto Parts. May not be able to get it from an auto parts store these days… it is nasty stuff! Aircraft Spruce might be a source.
The aerosol can version is fantastic but be very careful to use in a well ventilated area with no breeze and wear gloves and clothing that cover all skin. I used the stuff to strip a Tony chassis and it lifted the factory coating off with ease.