An old trick for underexposed or underdeveloped ("thin") negatives BEFORE you chemically, irreversibly, alter them is to hang the negative up, set up two lights (strobe or ambient) at 45 degrees Behind the negative, thereby backlighting it, and using a macro lens with a good deep lens hood,, rephotograph the negative on a fine grain film (I used TP until it went bye-bye, now would use Delta100). The back-lighting shows up the thin image and you can push the copy film to further increase contrast/density. And you haven't altered the original, which you can do later, as a last resort.
Good Luck!