Hi Alan,
thanks for the update. I personally would clean up the old piston as long as it isn't full of deep score marks. If there are minor ones, you can give them a sand to make it smooth again. The flat piston will reduce compression, by how much I don't know but better to reuse the old one unless you can exchange it the flat crowned piston.
I would also clean the carbon out of the end of the bore as best you can while you have it apart. Maybe try degreaser or acetone/carby cleaner.
