When you say the billet stem broke, do you mean the tubes cast into the helmet that lead to the billet, or do you mean the threaded section f the billet itself.
If the thread section broke off, you should be able to grip the threaded piece w/ some needle nose pliers and carefully unscrew it out and get a replacement.
If it's the tube leading to the billet, I'd resculpt with a bit of Aves epoxy, mask off and carefully retouch / repaint. With the alternating colors, you could tape off the tubes and repaint and it would look good as new. Allow to dry several days, buff to a glossy finish and readd the billet.