For the velocity stack... mostly for looks on these engines since they really don't spin fast enough to take advantage of them, they may work a little way up on top, but for the most part, they're just for looks.
If you really want to go fast, read up on porting, get a few spare cylinders and pistons to practice on and try out what you've read, and remove the restrictions from the intake and exhaust as much as possible. The stock NT style carb works surprisingly well and I've gone over 40mph with one feeding my engine. On the other side tho, the stock pipe is way too restrictive and one can benefit a LOT by something as simple as drilling a few extra holes into the end cap, or even better, getting an expansion chamber pipe that works for these engines.
The only pre made pipes I will recommend are the Snake pipe from Arrow Racing, and the SBP pipe from Sick Bike Products, these are both modofied pocket bike pipes but they work wonders... Otherwise, getting a small dirtbike pipe from a 60 to 80cc dirtbike like the KX, RM, YZ, or CR 60, 65, 80, or 85cc dirtbikes, cutting them and re clocking the bends until they fit will give the best power gain, but even an SBP pipe will give an instant increase you will feel as soon as you ride it the next time.
Compression is another way to boost overall power from just above idle all the way to top end. You can shave the stock head or get a pre made head that has far superior cooling ability over stock. The best heads for these are the Puch hi hi head, the Fred head, the diamond head from Jake's, and Venice Motor Bikes is also developing another high performance head that should do very well. You'll notice the difference just by shaving down a stock head, but may run into overheating issues if you live in a hot environment.
For the carb, the stock NT is a decent performer, but the Dax RT carb is a really good aftermarket one that will give a noticeable difference. There are also the floatless carbs like the Walbro or Tillotsons but these take some experience to get set up right. The Mikuni VM18 is also a good one that's not too terribly hard to tune and will give good power and same for the Dellorto PHBG series carbs up to 21mm, and the Dellorto SHA and their clones also work really well if on a budget or dont have a lot of time to tune.
Just avoid the CNS carb... this one does perform but it's a hassle to tune and you'll be constantly fixing leaks etc... I had one and put a stock NT back on it was that bad...
For starters I'd say shave down the head, drill the end cap of the exhaust, and use a free flowing intake and air filter, you'll feel that... Then moving up, get a Puch or Fred head, an expansion chamber pipe, and do some port work, or have some port work done, then time to think about upgrading the intake and carb... If you got a reed valve, there are more mods to do so they'll run at their best, just bolting one on you'll get better low end power but will fall short on top. There's some port work to do as well as putting a window in your piston to fix this problem, then it'll scream for ya...
