Looks like you are interested in running dirt tracks. You could run these thing on dirt, and they could be lots of fun, unfortunately they were designed for smaller European drivers. Fitting a bigger guy will be a challenge, as big seats are really difficult to install in a chassis, there is hope though.
Try to find a good used complete set up that already has an XXL seat on it, a 2001 Honda CR125 should be able to thrill you, but if not, a good TM ICC engine package will work very well. I wouldn’t run with the shifter kart guys, as you won’t be competitive, they are just way too fast, but you already mentioned you will only want it for fun, and for dirt tracks so no worries.
Look at your local tracks and clubs, kart shops and see if anybody has one available, also check your local CL, plus here in this forum there are plenty of them for sale at the classifieds all the time.
Please don’t be offended, but may be a good idea if you’re planning to shed some weight it will really help you, heck we all have the same issue here.
Anyhow, I hope this helps steering you in the right direction. Good luck.