Older round cylinder, if you are doing a restore then get a OEM piston that matches the cylinder, there should be a letter stamped on the deck of the cylinder that corresponds to the bore for a matched set.
If the original piston is not kaput just tired of neglect, clean it up and run it. Need two 39mm x 2mm - FG rings - then gap them to fit proper.
https://www.treatland.tv/50cc-piston-ring-p/ring-39mmx2mm-fg.htm
If you are getting a kit for the bike then get a new bottom end because the earlier AV7 engines do not have a oil journal for the mains and the bearings will not last.