Are those seatbelt auto-retraction mechs in those boxes behind the seat? Never seen that before, and doesn't look like they're from a car.
Interesting mix of tubing connections. Some are properly coped out for strong 90 degree welds, others are flattened for 90 degree butt welds. At least 6 tube ends are left round and open to the atmosphere.
Confusing, but overall the kart looks too nice to be any kind of completely homemade project. Maybe as you said, "customish".
Interesting steering setup also, to translate left-drive steering column position to equidistant center connection for tierods. Looks like perfect alignment match to factory A-arms, likely has almost no bumpsteer that way.