Ha . That looks almost as bad as the piston I'm running in my FA50 right now .
You could measure what you have and order an appropriate sized piston and rings .
Or simply clean what you have and run it .
Sand the high spots off the piston wall . Clean the ring landings and dome .
Run a quick hone through the cylinder to clean that .
Be sure that every thing is free of any debris and put it back together .