You can buy motorcycle gas caps new, but all they are are caps. It wou;d take a lot heavier fabricating to get it to work. the cap just kind of sits in a recessed hole in the tank and then bolts into som nut plates. It would be way easier to cut the tank then weld it onto your bronco. I was thinking of doing this mod myself but I found a 77 so I have door now.
You're best bet is just e-bay a couple tanks, or find a motorcycle salvage yard. You can pick up some tanks for cheap and the gas caps never get hurt in the accidents that destroy the bikes.