As far as a strait edge, you can use some sheet styrene, if it's thick enough. It's flexible, so you can follow the contours of the subject too.
Also, if the line is totally gone, or if you have a curve area you need to start from scratch, you can use the back edge of a #11 blade to get it started. And then switch to a needle or scribing tool if you like that better. This helps especially in small radii or curves.
Best wishes for success!