What ML unit is it for? I assume one of their Glock/hi-cappa/1911 hop units?
Remove the screw, take it to your local hardware store and find a replacement. Is it threaded or self-tapping? (My guess is threaded).
If it is for the units mentioned above, you'll want a counter-sunk headed. Take the whole unit (not in the gun...) to your local and dive into the buckets of various sizes and thread rates.
Oh no! They're after my cargo of sugar, tobacco and spices!