Well put the go kart up and the axle/etc is true/square, the bracket was poorly welded.
So, I did what any dad would do, took a 2 x 4 and pounded the bracket down.
It got slightly more true to the rotor, but I could not move it any more or the caliper when mounted would touch the bottom of the rotor, now there's minimum static clearance.
to do it right I'll need to bend it more, then grind away some of the mounting area, just maybe 1.5mm, keeping it square and what not.
For now, the parking brake and foot brake all work again, so my boy is back in business.