First, you will need to find a donor idle screw boss. (silver-ish piece in this picture.)
Next, cut off your existing/broken boss from the carb. Make sure to cut it straight and flat. Once you get your old carb prepped, take your new idle screw boss and and do a little test fitting until you get a good, flush fit.
Next, drill a tiny pilot hole into the bottom center of the boss, and then drill and tap a corresponding hole into the carb where you cut off the old me. The arrow in the second picture is where you want the set screw to go. I forget the size set screw I used, but just pick anything that will fit and that you have a tap for.
Put some jb weld on the new piece, push it onto the carb, insert the set screw, and then go BRAPPPPPPPPPP!