Had this problem not too long ago. As for cleaning the dirt out, use a q-tip and go to a local computer store and get a can of air. This is not a joke. They have spray cans for cleaning dust out of computer parts, works great on cleaning the carb. As for the screw, look to see of the needle valve bent as a result of the crash. If so, try to bend it back and take out the screw, otherwise as the others have said, it went on, it will come off, just take your time and think it through. What type of engine is it?

Best of luck

Victor