The racing carb is going to give you a bit more kick, but the 80cc bbkit would give you a hell of a bump. You would need to make sure you have a decent matching racing exhaust to go with it though. The TK exhaust for the 50cc is quite nice btw.
Now, if you push the envelope too far, you are going to need an upgraded final drive shaft... not an easy thing to get and could be pricey. If you haven't touched the variator, rollers, or clutch... that's going to be your next area to upgrade, possibly prior to the bbkit. You could get an 80cc engine to top out close to 55 or so. Just curious... what other mods have you done?