I just sold some late 20's radiator ornaments and both of them had the internal threads basically wiped out.
I thought if somebody was going to repair these that a pipe with inside threads could be used. It would have to be turned on the out side so it would slip into the mascot.
Some kind of epoxy or
would have to be applied so that it would stay attached.
Other alternative that other poster suggested would require having the radiator filler removed and capped by a rad shop, then an elbow for a radiator cap installed.
To follow up on what the other poster said , with bolt and an aluminum or steel cross brace the mascot can be attached to the radiator shell.
I believe the 1931 or 1932 radiator mascot has been reproduced and would be the best choice if one wanted to stay with stock and avoid all the hassle of trying to repair the original stock one.
mike lynch