Always a chance you could put a Nikki carb on it...have no idea what all you'd have to change, but doubt it would be a drop on. For sure the air cleaner assembly is different, cables should work (went from a B43M w/DD carb to a P220 w/Nikki carb). Not sure about the intake manifold.
Is the carb still working or is it leaking due to the missing screw? Do you have a picture with the top off? I do have a lower on a parts machine ('75 446), but not sure how serviceable it is.