Stick on a cowbell. Everything needs more cowbell.
Lol THIS GUY knows what's up. For real though, the best bang-for-the-buck upgrade
for me was a "racing carburetor". You know, the deal you'll find on pocket bikes. I got one that included the proper throttle cable, 5 different jets (technically six) ranging from 60-80, and never looked back at the NT garbage again. Don't get me wrong, if simplicity is your goal the NT wins every time. BUT if you know how to tweak a carb, you can really open these engines up and let them breathe. Plus the option of changing jet size will really help your bike run better depending on where in relation to sea level you're riding it. The NT has a 70 jet if i'm not mistaken, and other riders in my area swear by 62 jets, so there is a little difference there. Also, forget about leaks with this carb. Delrin bushing keeps everything air-tite.
Oh yeah, if you do go with this upgrade let me know before hand and I'll let you know where to get a deal. I got mine for $40 shipped to my door (which only took a day and a half from the time I ordered it) with the cable & jets. I'm told that if you pay more then $35 for one of these and it doesn't include jets and cable, you're getting ripped off. I'm also not trying to sell anything to anyone either, so please don't think that. I'm just speaking from personal experience.
Edit: I said technically six jets because there were 5 bags labeled 60, 65, 70, 75, 80, all containing a jet (in the package that arrived). When I took the carb float bowl off to install my preferred jet, there was already one (of unknown size) in there.