You don;t wanna fuck with your paint job. Understood.
I sold a trike that had a sporty tank (back when sporty tanks were 2.2 gal) The fucking thing was sold and the tank bracket cracked riding it back to the shop. Tried to weld it up right quick. Though maybe I could touch up (underneath). Popped a big chunk of Bondo off the top! several inches fromteh heat affected zone, had to get it to bare metal, tin the ding with brass and lead it up with solder, then spray can the whole thing. Different color close but no cigar. Was up all night. dude showed up the next day, wanted a fuckin discount. He didn't get it.
Was looking at the bungs easy enough to make if you got a lathe or even a drill motor and a pipe tap
doesn't have to be round. But you might have to add a doubler to the sheetmetal around the hole if the weld's fucked it up.
Pitch gague and calipers ought get you the specs you need to fnd out the nominal size of the bung though. be amazed what you can fnd with google.