How about enlarging it to film, then retouching before you contact print. Or if you enlarge to paper, then you can even more easily work on the paper negative, with a pencil on the back side. Havign to go through paper negs won't give great sharpness but who knows it may work. And then of course there is the digital negative process, which may well be your best option here.
I have a similar issue with one of my favourite negs and my plan is to enlarge it (using edwal on the negative) to 10x12 ilford ortho +, retouch if necessary, and then contact print once i have a good duplicate. We'll see how that goes...