I wouldn't recommend the speed carb either, and if I had one I'd swap out to the NT because the speed carb has the extra fuel valve that I've found to be problematic as there's no filter between that valve and the float needle and seat valve so rubber particles from the fuel valve can shed off the inside of it and clog up the needle and seat valve or the main jet.
The NT carb can perform very well on these engines once jetted and tuned properly, and their simplicity makes them easy to fix and very reliable. Even with some really good mods to your engine like porting or a pipe the NT is still plenty enough carb. The speed carb is pretty much identical except that it has that fuel valve I don't like and I think it may be 1/2mm bigger at the slide, which won't even be noticeable.
A properly tuned NT carb with a good pipe will definitely wake up your engine and I'll also agree about switching over to the Opti2 oil and mixing it at 100:1 as this oil is designed to run at that ratio and plenty safe, you'll need to jet the carb accordingly but you'll also pick up a little more power from the higher mix ratio as well as a cleaner smoke free exhaust and a lot less opportunity to foul the spark plug. I get the Opti2 oil from amazon.com and it comes in single mix packets or you can get it in quart or gallon size bottles but I like the quart size with the metering chamber so getting the mix right every time is easy. http://www.amazon.com/Opti-2-34oz-EZ...keywords=opti2
The combination of running the Opti2 oil, a better spark plug, an expansion chamber pipe, and tuning the stock NT carb to run with the pipe at the new mix ratio will net you some impressive gains, The SBP pipe is one of the best "off the shelf" pipes out there and costs about $80 but even those cheaper banana pipes will give you a good boost and with that combo you should be able to stay under $60 for everything. Of course, the SBP pipe will make better power if you can get it, but if not, a banana pipe will do the trick, and if you can weld, a modified pocketbike pipe or a modified KTM 50 or 65 pipe will work really well too.