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, no snorkel, inlet enlarged, 118/40/NBZT "Thruxton" needles/1 shim/3 turns, Ikon 7610s, Ricor Intiminators, Dunlop GT501s, D9 gauge panel, tachometer.