how do i find the exact size? i cant even find the original size 72.45 that is stamped on the top of the piston

 

do you have a caliper, you can measure it across, probably best to take it and the cylinder to a machine shop, they will measure out the cylinder and tell you if its egg shaped or worn more on the bottom ect, how much it needs bored to true it, then you can order the proper over size piston and rings they will bore the cylinder to match the piston