I wouldn't buy the speed carb, lots of people have lots of luck with them but to me it dosen't look like a substantial improvement over the NT that comes with the kit. but that said, I had really bad luck with the NT that came with my 2-stroke, maybe it was just bad luck.
I bought a dellorto SHA clone from pedalchopper and that carb treated me great.
the SHA has a wider throat than the the NT was so when adjusted properly you get a higher volume of air/fuel mix going into the engine, this gives MO POWAH! also the SHA clone I got had the choke lever on the end of a pull cable so I could put it on the handlebars, super convenient compared to the choke on the NT carb.
ultimately what carb you want is your choice and there are plenty of choices to be had, if you wanted to get an intake adapter you could get a bolt on carb like a mikuni, those are fairly well regarded (havent used one myself though)