Just had to do this not too long ago...best option would be to find a junk carb and cut off that piece to use on your exisiting carb, or possibly call ed at Full Spec Carbs and ask for the throttle boss piece that he cuts off carbs. I know a few guys here had htem just hanging around.
Step 1....find the throttle boss piece you are going to use. It should look like the silver part in the picture below. Prep your carb my cutting off the broken throttle boss. Cut it as straight as possible. I just used a dremel with cut off wheel...worked perfectly.
Drill a hole through the new piece into the carb. Tap the new hole in the carb for a small set screw. Mix up some JB weld, and put it between the new piece and the carb. Install the set screw and let it cure, and youll be good to go! Ive been spinnin and floppin all season and my fix is still holding perfectly.
Here it is fixed all together.
Let me know if you have any questons...glad to help!