Labor will be the expensive part, if you do it yourself it will be cheap. You could weld a new bung right on top of the one that's there. You could drill and tap for next larger size thread. If you have to pay someone to do this you might be better off buying a new cylinder. Hard to tell what size that is, but I would guess you could get one from Surplus Center for about what the labor will cost you to either weld or machine on that one.