We have built and rebuilt lots of karts. You can do it if you have a pipe bender and are a decent welder and it takes a bit of knowledge of karting. Mild steel, chrome moly whatever. If you know someone that has a decent kart ask to get a pattern. Our jigs usually start out on a sheet a plywood, the pattern gets put down, and the pattern gets modified from there depending on what you want to do with the kart. There are also plenty of places to source parts from and such. If you are real good they will perform just as well as some of the stuff you can buy. I’ve won a track championship on our 2nd generation garage made oval kart, and our rebuilt and scratch built vintage kart frames are as good as anything out there.
We now have a fleet of oval karts and other karts. And yea we have a backyard track that we run on also for fun, in addition to actual tracks. Clay even a silty clay makes for a decent pack whether oval or otherwise:
My rebuilt 1968 Bug Sprint, it was pretty much a wreck: