Change the Piston!
I cannot stress changing the stock piston enough! I bought a 3 year old 220 that had 5000 miles on the clock on the original piston, I tore it down shortly after purchase to give it a check through for peace of mind and found it to be in good shape. As i had the original spares kit i fitted the spare piston and rings etc, thinking that would take care of the top end for a good while. 10 monthes later (and 1800 miles of mixed riding)whilst on a long distance trial the bottom of the piston fell off whilst the engine was idling, destroying the barrel lining. I had to remove the engine from the frame to clean out all the aluminium debris, (but then decided to feshen up the bottom end anyway) I also had to get the barrel replated as well. So i could of easily had a bill from a dealer for approx £500 ($900) to fix all this if i was not mechanically minded. As it happened it cost me around £300 ($540) in parts to fix it.
So based on my experience if you have a KDX220, change the piston to a forged Wiseco one as soon as your budget will allow. I wish I had!!!!!!