Lacking specifics here....is the cluth slipping or simply not engaging at all? Why do you suspect the cable, instead of the clutch, itself? At that age, I'd suspect the clutch is likely due for a rebuild.
As for the fender, replacement is the best way to go. Replacement plastic might be hard to find for that year but I'm sure it's out there. There is such a thing as "plastic welding" but likely, only an automotive body shop is likely to have the equipment...even though a plastic welder only runs about $150-$200. I can't vouch for the quality of the repair, though...may not be strong enough. For a quick redneck-fix, get a piece of .040 aluminum (or a piece from an old plastic fender), insert it into the underside of the fender and rivet it in place, to splice the break. It won't last long or look trick but it'll work, as a last-resort, temporary fix.