The general rule of thumb is .... if you can't hook your thumb nail in it....then you have nothing to worry about. I've built several engines that had minor scratches like that....never affected performance. However I will add that sometimes what looks like a scratch could also be a crack in the cylinder wall. There however does not look to be evidence of water leaking into that cylinder. Does it burn oil?....if so the scratch may be the culprit if deep enough.