Bring your cylinder to your local shop and they should tell you what size they will over-bore to so you can order the correct sized piston...better yet have Jack do it and you'll know it was done right...

 

Cant find a shop that will bore it for me so ill be ordering one just to get him riding again he was riding on a track back in November and he stopped and the person behind him didn't see him and ran him over they where on a 400 so he was badly injured when he woke up in the hospital the first thing he asked was if he could go home and ride so that shows how much he loves it he was in a full arm cast for 2 months and had 3 surgeries I have a 400 king quad that I let him drive me around on but its time to get him back on his own quad it only runs 25 mph and after driving my 400 he wants his to go faster hopefully a new top end will speed it up just a little

 

I might have a bored cylinder here to swap quick like.
As for more speed from boring.. prolly not.
I could do a quick stage 2 kit..

 

After looking at the cylinder head in better light I can definitely see a lot of wear LT80 I was going to send you an e-mail to further discuss what you mentioned in your comment