Its a simple metric threaded screw , I know a 10 mm wrench fits the head if it were a regular bolt. But try a few metric sized bolts till you get one to thread into the carb.
Once you have found the correct threaded bolt, just simple grind the head to a cone shape , add a small spring to it and you have yourself a new ideler screw.
If you tend to loose your idle screw alot, then use a short cable lead from your fishing tackle box and wrap it around your idle screw so it stay in place, then find a nut or bolt some where to locate the other end , so that if it works its way loose again it will still be tied to the engine by the cable you just added and then you can simple reattach it .