Bigger carb will not help a stock setup, if you're tuned and timed right on that 15 bing then you're probably getting as much as you can out of that setup. Since correct timing on 2 speeds can require notching of the the stator plate slots to achieve, I'd start there if you're looking for gains on your current setup.
You could also get a cheap used aluminum stock maxi-style 2HP cylinder and piston and open up the ports and pop that on with that same setup. There's some good info on porting stock cylinders in the Wiki. That would wake it up some and it's a fun learning experience before getting into expensive kits.