It's a number 0 Philips screw in there. If it's the knob that's broken, you can buy it separately.
Edit: I see that Ruger beat me to it. So there's your answer. If the stem itself is broken, I think that you're out of luck.
2005 Bonneville Blue 790cc, AI removed, Staintunes RC, Uni filter, no snorkel, 118/40/NBZT "Thruxton" needles/1 shim/3 turns, tachometer, Ikon 7610s, Ricor Intiminators, Dunlop GT501s, D9 gauge panel.