Donald's solution sounds quite workable. I only have experience with pencil retouching. If the scratch has rough edges, it can smoothed slightly with dental pumice on a q-tip (dental pumice only - anything else is too coarse and could cause more damage). That will also roughen the emulsion slightly to take more pencil. Most modern thin emulsion films have very little 'tooth' and don't take pencil very readily. If you go the pencil route, retouching fluid may be applied (available at B&H - about $4) to make it work better. Retouching fluid is an oily substance that will stay on the negative - it needs to evenly applied with a cotton ball on the whole negatve to avoid any missed spots that could affect printing.

And yes - do experiment with scrap film before trying anything on the negative you are trying to rescue.

Here is a tip to help smooth the retouching when printing - get a piece of cellophane (like the cellophane wrapper on some cigarette packages). Then starting at one corner, crinkle it finely through the whole piece so that when you flatten it out, you cannot see clearly through it. Durint the exposure, hold it a few inches below the enlarging lens and move it in a circular motion for about 1/3 the exposure. This will help blend and soften the retouching without seroius effect on the overall apparent sharpness. At more than 1/2 of the exposure, the print starts to get really soft.

Good luck.