I would get a new top end gasket set because you will have to pull the head and cylinder off anyways. With the little experience I've just done with re-doing the top end of my '98 yz125, you might want to check the tolerances of the cylinder bore and the piston size to make sure that you won't have to have the cylinder bored. Make sure that when you get the new ring(s), place it in the cylinder and square it up with the piston and then check the gap clearance. File down the end of the ring according to the specs of your manual and don't forget to smooth the sharp edges of the end. If your piston doesn't have any stress cracks in the skirt and there isn't too much carbon build up that you can't clean off, then re-use the one you have if you need to get back out and ride. I would replace the piston myself because of peace of mind!