Rocketsled,
If you are looking at a good buy on a used kart, Garett P has two CRG Rotax packages for sale with additional spares. You might attempt to meet him at a track and take one for a test spin. I’m not sure how you could stretch the kart to match your height. There are plenty of smart tall karters who will provide this info.
If the Rotax engine has a “passport,” you could run the RMax challenge Master class at CRE (min weight is 400 pounds). If not, run IMI’s club Master’s class (min SKUSA weight was 395 pounds and no seal required). You should be close to the minimum weight without adding weight to the kart.
Finally, if you just want to bang tires with your friends, the Rotax could let you do that. If you want a little more reliability, look at the Briggs and Stratton Animal or World Formula. It is a slower kart, but has plenty of torque to get off the turns.
I hope you fulfill your racing dream!