No, I didn't show him the plug. Forgot to mention to you guys, that it's an old plug, and the pebbly surface was on it already when I put it in the bike. The current cylinder/piston usage did not cause that.
Got a new piston ordered, once it's here, will drop the cylinder and piston to a workshop to be fixed and I will make sure it will get within the piston/cylinder clearance specs.
Also picking up an '02 kx250 tomorrow. Will use that for a kx500/250 conversion project once I get a bit more money and find a suitable donor bike for the engine.