i believe that if the scratch isnt deep enough and does not cover all the piston(up to down) you ll not have problems. Try spraying some wd40 on top of the piston (from the glow plug hole) while engine is assembled(without pipe), put glow plug and turn it over slowly.Check for any bubbles on the point of the scratch or if you can hear the air escaping from this point while engine is on compression.
hope i made myself clear.
Serpent 720 09 mega zx 12 dsII+, Serpent 733 nova 353 tuned/sirio XXXV
Futaba, Graupner, Hudy, RB fuel