That has to come off to clean the carbs. Behind that plate is the high speed check valve. If you hit it with carb cleaner... it will get brittle, and crack. Also... you can't get to the main jet with it in the way.
If there is any bit of the screw head left... you can try a hand held impact. I use one to disassemble any carb a touch. If it's completely gone... you will have to drill the head off, and remove the plate. Once it's out of the way, you should be able to grab what's left of the screw with pliers, and twist it out.