There is several causes. I have built race winning auto engines for many years. There is always some wear on the thrust side of cylinder walls that happens during break in. This can be made worse if not enough plateau on the honing or it just to rough. I can’t tell a lot from your pictures but looks like there is vertical line on both sides of the walls. The lines are below the top travel of the ring stack ending at about the piston belly. So not ring clearance. Looks pretty uniform, so not trash. So we are left with heat , pistons to wall clearance, or lubrication. Doesn’t look like heat, or at least not enough to cause molten metal transfer! Heat does close up wall clearance so that is one thing has to be considered with wall clearance. Piston to wall clearance can be tricky, different piston materials , manufacturing, engine type and use, may need there own clearance. Forged piston expand more with heat than a cast piston,, for example. Lubrication can be lack of or washed down . If you didn’t clean any of the pistons (in your pics)and looking at the walls above the ring pack area, doesn’t look like to much fuel except one cylinder has a little soot on on top of cylinder wall. I am not a kz expert we just played around a lot with them in our dealership and raced them mostly on the street. One thing I discovered on a kz was on a dry engine ,oil takes a long time to get from oil pan to pressure in oil galley(not just turn off light but 2-3lbs on a guage)on first start. So I ended up priming if a rebuild! I always used gm E.O.S on cam and engine bearings and tranny fluid on cylinders and pistons back then! So if oil problem ,,correct and run it! If clearance problem,, hone to correct tolerance and run it! You can lightly sand pistons belly and skirt with 600 wet dry sand paper to knock of any high spots from scuffing ,,if need be! Make sure you always have air moving over the motor when it’s running (fan or riding) specially before it has some miles on it! Guess what I am getting at is looking at your pics , the motor is not f-Ed. Just saw the movie , side clearance and piston rocking doesn’t mean anything. There is no skirt on the side of a piston and some skirt designs can rock more than others.The ring stack has a much smaller diameter than the actual piston size. The piston belly (fattest part of the piston 90 degrees from piston pin )is the critical place to measure for clearance. You can check clearance by sliding a piston (long) feeler gage between piston and wall at the front of motor. This isn’t the most accurate way to measure ,but it’s would be close enough for this application.