Get some shellac sticks from Brownells. You can get a shade to match your stock and with a little practice you can fill the scratch so no one will see it. I've done this many times with old rifles, there is no need to sand the original finish and if it's got a bit of dirt and goop on the stock so much the better. You can fill the scratch, contour it with a fine file to match to good wood and blend it with the existing finish/goop with a bit of what ever solvent works. I've done a number of stocks and after a year I can't tell where I did the work much less anyone else. The dark and medium sticks work best for me, plus it's an old technique, no one can say when it was done if they do happen to find the work. Beats trying to refinish an entire stock and loosing all that patina and originality.
Best.
