The stock cylinder is plated. You can't just run down to the local machine shop and have it overbored for $40 and slpa in a new piston and rings.
If the plating is not worn out, then the stock size piston and rings are used. If the plating is worn out, it needs to be replated.
A guy who posts on this site and has a jillion hours of experience (even for the World GP races!!) told me that the 1989 - 1994 KDX200 can not be overbored without losing the power valve system, since the valves are very close to the cylinder. I would suggest you leave it stock.