Why do you want to change the piston? This is done when its worn past its limits or has been damaged. On a 2stroke when its replaced you go up a piston size. Generally .5mm sometimes more. I had to go a full mm on my TDR when the powervalve and piston came together with the piston ring. What is the problem you have, I'm guessing you may just have excessive wear even then you will need to rebore. You may get a bit extra with a set of rings but, again. My TDR was brought low by the age of the piston and the ring peg failing to stop the ring rotating. If the bore is clean and not out or round you may be able to use a .25 increase but it is tiny, barely more than the piston tolerance of .125mm either side. You may be able to use it but the full 2.0mm bore may be better for long term reliability. As will using GENUINE gaskets, don't believe the bull, anything other than genuine yam or Athena head gaskets will fail. The only names repeated for these bikes for replacement pistons that will stand the use are Mitaka (best price), Woosner and Genuine OE Yamaha, (gold plated I think). I fitted mitaka in my TDR250 on the recommendation of everybody I asked pretty much.