If you try removing the screw and re-tapping the hole remember the screw is probably Metric. It's hard to tell why the screw broke off. Siezed, damaged in prior crash, heck- it may have been CA'd in for all we can tell from the pics. As for having a machinist take it out - a lot cheaper to replace it with a better carb or at least a new one like that.

Part list
http://www.osengines.com/parts/xr12331.html
and carb:
http://www2.towerhobbies.com/cgi-bin...00&FVPROFIL=++

Good luck

http://www.osengines.com/parts/xr22381000.html