There is absolutely nothing wrong with the stock carb, just open it up, put a large round 110 main jet in it, shim up the needle and turn out the idle mixture screw to 1 3/4.
Jet:
http://www.ebay.com/itm/171069472877
Procedure:
Note: I used a center punch to indent the screws and then tap on them to back them out rather than slot them with a dremel.