What is the carb number. IE WT... or WA...? According to my formula you would want to try a carb in the 11 to 12mm range which would give you a venturi of 28 which will be cast into the carb also. For a carb arm you could use a steering nose gear arm. The arm would sit on top of the throttle shaft and be held in place by a socket head screw. There are many other ways too.

TIm