If you can get it out, someone can probably come up with another screw. I have a carb here somewhere off that series of saw with a broken off screw but don't know offhand if it was the L or H screw. I tried unsuccessfully to remove it (not a real valiant effort however)
If you can find a piece of metal tubing that fits around the diameter of the screw, you can make a drill guide to help drill it on center...