the original carb is pretty small, if you're going to spend the time putting a kit on you might as well go for a larger carb. I am not a fan of the hobbit carb. it's a butterfly carb, not a slide carb like the stock express. either a SHA or PHBG would probably be better for you. You're gonna have to get creative with the intake though... Check this thread break>
You should probably get a pipe too, the stock one isn't the greatest. Treats has either the Jamarcol (cheaper) or the Proma (expensive / out of stock) listed on his site.
You could run stock pipe & carb with an upjet, I know Graham from Milwaukee put a 70cc DR kit on an express for another one of the Milwaukee guys, and it's definitely a ripper, but probably has vastly more potential with a carb & pipe.