While checking the top end, I think the piston rings are worn enough to replace them. The ring gap on the top ring is .75mm and the 2nd (or bottom) ring is about .45mm. The service limit is .63mm.
I didn't find a manufacturer on the piston, but it does have a "B" and and arrow on top. "8K" is molded inside a 5 point star on the inside wall with a "1" above it. Does that sound like a Kawasaki piston?
The the stock pro x(stock) pistons come in three different sizes they are marked with a,b,or c. The one you have is the stock piston. There is also a letter marking on the top left of the cylinder if my memory serves me right.