Hmmm..that looks like it would be a lot of filing to just clean it up. The problem now is that because there is an angle on the corner edge of the piston it will act like altering the height of the port changing the timing. That big angle will open the port sooner and close it later. Having mismatched port timing will most certainly hurt performance in a substantial way. It would be a much safer choice to replace both pistons and ring sets at this stage to maintain equlibrium between both cylinders. If the piston only had some minor marks on it, but the edges were still for the most part quite decent, then yes you can take emery cloth to smooth out minor scuff lines when in a pinch. But if that were to happen to my engine, there would be a full top end rebuild under way. In the world of water fun, don't take chances. I have had to tow guys back due to empty fuel tanks, bad batteries, or carburetors that needed to be rebuilt 4 years prior. If they were out there on their own, the swim to shore would have been brutal providing boaters didn't clip them from being so nearly invisible on the water. One little engine failure that could have been prevented by doing a proper job in the beginning could mean being a 20 inch tall white cap speed bump to a bow high boater.