Steve how bad is the scratch? Can you post a pic? See if you can feel it with your nail maybe you can rub it out with a filter pad very carefully and see if it buffs out. You never know it could just be some schmutz and will come right off.
Basicly scratches in glass tanks are there to stay as there's no easy way to buff them out or get rid of them. 9 times out of 10 it is easier to buy a new tank than deal with the amount of work and effort involved in trying to remove them (unless you have a large tank or an elaborately drilled one).