[embee]

I certainly wouldn't try to drill it out freehand. I'd strip the carb and file off (or use a milling machine if you have one around) the remnants of the screw until you can define the centre. Then set it up on a pillar drill or mill (angle plate etc) and use a spotting or centre drill to start the hole at the centre of the brass and go bigger a bit at a time, effectively shelling out the screw. Not a 5min job.

Unfortunately the thermal expansion of brass and alum alloys are very similar, so heat of itself will be of little use. Soaking in a proper penetrating oil might help but I wouldn't hold my breath.

Alternatively leave it alone ………………….