Don't worry about the mark on piston. Do replace the valve. With the new valve in place, squirt some liquid in the exhaust port to see is if the valve seat is not damaged from the screw. 
Don't forget to replace the oil seal on the exhaust valve.(there is no seal on the intake)                                                 CT

Last Edited By: The CT Shop Mar 1 11 8:41 AM. Edited 1 times.