I have some experience with motion-picture negatives. I think the best approach you can make is to produce contact duplicates. That stresses the negatives the least and yields the truest restoration step. You will have duplicate positives of equal size of the originals. From these you can enlarge, print, continue duplication work, scan the image, whatever you want.
The first cut is the deepest.* Contact dupe positives should be made on microfilm. You can buy microfilm in various sizes. Bring it together with the negative under a not too thin glass pane, shine light on. You can try out ultraviolet, violet, blue through red or infrared besides white light. You can use a point light source or diffused light. You can use liquids to prevent Newton rings, glycerin for example (a water-soluble alcohol) or microscopy oil. Before development of the exposed microfilm you simply rinse it in water or wash it with a detergent and a synthetic sponge (doesn't contain sand grains), in the dark, of course. Feasible. The trickiest part is to develop the microfilm to an acceptable contrast overall but that is also feasible.
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*That comes from turning steel bars on a lathe where you need to remove a scale skin with the first pass.