Yep, that is the idle circuit fuel screw. If it is missing you will get a little air (and no fuel) through the idle circuit. Not a good situation. It should come with a spring, O-ring, and a little washer. This should not cause plug fouling.
Have you got a Honda dealer anywhere? Parts should be readily available. I have found with carb parts that it is best to stick with OEM or original carb manufacturer parts.
Click on Service Honda (immediately to your left).
You could maybe get by with a salvage idle circuit screw, but if you get to the needle and needle jet, go for the new stuff. The needle is the long metering needle that passes through the slide. The needle jet is in the carb body and requires removal of the main jet carrier. It then knocks out from the top (through the slide bore) of the carb.
You sound a little inexperienced. Remember that when doing any work on a carb (or most any other part of the bike) you must be patient, methodical, and METICULOUS to avert complete disaster.
Murphy was an optimist!! Good luck!