As I recall your old piston showed some scoring on its outside. That means some aluminium from the piston will have stuck to the bore of the cylinder. The bore needs to be extremely smooth, for the piston and rings to run up and down it without damage. It needs to be honed just enough to remove any rough spots. This can be done with some wet-and-dry automobile paint-rubbing abrasive, or with metal-polishing abrasive (either cloth or paper backed). Around 220 grit would be OK, if there is anything to be removed. If you are confident there is no aluminium stuck on there, up to 400 grit could be used. My preferred way to do it is to use a round piece of wood of just a touch less than bore diameter, drill a hole through it lengthwise so you can put a rod through it to spin it in an electric drill, and make a fairly shallow saw-cut radially into the wood along the full length. Feed the end of the sheet of abrasive paper into the saw cut, then wrap it around the wood. Cut the paper so it just makes one turn around the wood, without overlapping. Feed it into the cylinder, with some kerosene for lubrication. As you start the drill running, start pumping the lap up and down in the cylinder so you are polishing the bore with helical motion, not rotary motion. Keep trickling kerosene into the bore to carry away the grinding dust, or it will cut grooves in your bore.
