Expert? That is one abused piston. It has been drawing a lot of debris into the combustion chamber. The combustion has been blowing by the rings. The piston rubbing the debris against the cylinder, you have been honing it all along. I would double check the bottom end, this gritty fuel has been washing the bearings. Any up and down movement in the rod or at the crank ends, its shot. Measure the thrust washer clearance with a feeler gauge. You need to measure the bore, at different elevations and degrees around the bore, where the piston rides. A bore gauge, and inside micrometer are good. Heck, they probably can do it electronically! In a pinch, with a new piston, place it in the bore and measure the clearance at the skirt with a feeler gauge. The head is probably good. I think the carbon on the bottom of the piston is from the blow by. 300 to 450 dollars should cover the parts to freshen your top and bottom end. Re-plating, call Eric.