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I would definately see Chi Quin Optical (Isaac) first, he will be able to advise what needs to be done, and he knows how to safely remove old overcoating if it does need recoating. If not done properly, removing a SIo2 overcoat can ruin the figure of the mirror as the underlying glass itself is damaged.
If the scratch on the mirror is in the middle, it wont affect optical performance at all, as it will be under the shadow of the secondary, and even if not, a scratch wont affect it much anyway.
Some decades ago in the US I think, a Uni student had a breakdown and opened fire with a gun at an observatory telescope mirror. The bullets chipped the glass, but all they did was paint the chipped areas with black paint to stop unwanted reflections, and kept using the scope as normal, the light loss neglible.
Scott
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