Hy 61lorrie, I not positive, but I don't think you can remove any pistons from the bottom. Yes you can replace just one piston, if you get the correct sized one. If you're going to install a different piston and new rings, you should cut the ridge off of the top of the cylinder, so the sharp edge of the new ring doesn't cut into the ridge, possibly breaking the ring or one of the ring lands on the piston, hope that helps.
