i fixed one with jb weld. normally i'm against using jb weld for anything, but it worked.
drain the tank and use a wire wheel to grind off all the paint and rough up the metal around the stud.
thoroughly clean the area with carb cleaner or something. make sure it's completely clean and completly dry.
mix up some jb and slather it around the stud.
let it dry over night, or in the sun, whatever, but make sure it's 100% cured before you reinstall it.
cut up an old innertube and wrap it around the frame where the tank mounts to give it some cushion.
tighten all the nuts securely, but just enough so the tank won't move.