Northstar pic is a ring issue you can see where gas's have gone past the 2nd ring .. Splattering on the dome is ign ping. ( for what ever reason) you can see where it is pitting in a few areas. High heat some detonation that will knock it loose in certain areas. Air cleaners that introduce mist back into the engine can be the issue. However with 90+% of all air cleaners doing that very same thing the odds would have to be that all pistons would like like that. That is however not the case. With poor ring seal you have increased crank case pressure which will increase the amount of oil carry over = more oil mist going through the system into the engine
What we see is that many of the cylinders are less than round & straight and outside n spec though out the entire area the piston is traveling. Rings have a hard time controlling oil when that occurs. Simple bore and hone will solve the issue. I have countless photo and measurements of 110 cylinders. As well many that thought the air cleaner was the main issue and routed the breathers to the ground. Oil still was baked to the piston dome. Not to say that is always the issue.
We have also found that just about all will mist pretty good with 5000+ rpm ranges in stock form. Bore and hone and the crank case pressure is reduced and no more oil mist issue's that end up on piston domes.
Damage to the piston skirt as pointed out is another piece of info that points towards improper ring seal. More info nothing more, being you are going to bore and hone with the new pistons your issue there should be solved.