You could copy the negative onto lith film develope it relatively hard and copy that again onto another sheet of lith film.
This sometimes works for me but is rather time consuming. Or if you backlight the negativ with two lamps at a flat angle and photograph it and develope it hard you could get a new negative.
I do not know what this process is called but this used to be done. Myself I have never done this. But I am planning to in the future.
Selenium toner does not really help with very thin negatives.
Does Chromium Intensifier still work with negatives which have been selen toner already.
Does anybody know?