Tom,

If the screw has broken flush with the top of the crankcase, you might be able to get it out if you can get hold of a really thin file (you probably need .02" or less), and then file a slot across the end of the screw.
You should then be able to remove it with a jeweller´s screwdriver.
You will have to file a bit into the crankcase too, but what the heck, if the alternative is to scrap it?
You can get spare screws from PAW in England.

The photos below show a broken needle that I got out of a ST carb with this technique, maybe you can use it too?

Tomas H.