best bet would be to remove the inlet throttle boddy and get it in a vice with saftjaws (not too tight as u may deform the ally, and leave it soaking with some plus gas or wd40 etc over night and then geta good fitting parallel blade screwdriver down th ehole and give it a whack on the end with a hammer a coupl eof times, this usually does the trick, next move woulbe be "get the gas out" geta blowlamp around the casting where the screw is and warm it up, being carefull not to melt the ally again, loads of wd40 down the hole and try again.