Available aftermarket carburetors

Speed Carb 15mm $25 - some have a spring loaded float valve which is better than the NT's rigid valve. It's lacking a low speed circuit (idle jet + air screw) and different needles are not available (set the clip to the 3rd/middle position. Main jet size needed to be #72 Dellorto jet for my 48cc reed engine.) I only recommend this carb for 48cc reed valve engines. The stock NT is better for piston port intake engines.

Dellorto SHA 12-16mm - other than changing the main jet this carb is non-adjustable. I had one and had to do lots of mods to it for it to be jetted right. I don't recommend it.

Keihin Carburetor Clone - clone of Keihin PTF 16mm. Pilot/Main jets are available but no needles. Intake bolts onto intake. Sold mostly for 4 strokes so maybe the needle jet is of the 4 stroke design.

Mikuni VM16 $85 - Clamp-on size is 20mm. Main jet 70, idle jet 25, Needle Jet D-8. slide cutaway 2.5, Needle 3DJ3. No needles are available for it so I don't recommend it.

Dellorto PHBG 17mm AS $99 - good choice. Clamp on size is 24mm. Comes with atomizer 262 AU, idle jet 50, main jet 78, starter jet 60, needle W7, slide 40 (4mm). The leaner W12 needle would work better for an engine with piston port intake.

Mikuni VM18 $80 - my favorite. Clamp on size is 23mm. Comes with #30 idle jet, #80 main jet, 1.5mm cutaway, 4F18 needle (use 4D8 with piston port), O-0 needle jet (change idle jet to #22.5 for reeds or #10 for piston port) Click here for the $17 CDHpower G2 Reed Valve with 23mm OD for Mikuni VM18.
Jamon Howell sells this manifold for the VM18:

OKO Flat Slide Carb (19, 21, 24, 26, 28mm) $110 - 34mm OD Spigot mount. Don't get any of the cheaper imitation carbs ($28) because they often have problems due to sloppy manufacturing tolerances and sometimes they won't even accept the normal jets for it. Oko can select the jets they think will work with your bike. Needle kit source

Dellorto PHBG 21mm CS clone $65 - Clamp-on size is 24mm. Click here for an original PHBG21 since I don't really recommend buying a clone carburetor since they usually have bad manufacturing tolerances.

Mikuni VM24-512 $113 - Spigot mount, Pilot Jet 40, Main Jet 210, Needle Jet N-8, slide cutaway 1.5mm, Needle5L14. The inner diameter of the connecting hose needs to be 24mm for soft rubber or 26mm for hard rubber.

Dellorto PHBL 25 BS $100 - 31mm spigot mount (use $10 rubber connector) comes with #70 idle jet, #102 main jet, D22 needle (I recommend replacing this with D21 which is leaner).

Mikuni VM26 (#8074, the 2 stroke version) $77 - Spigot mount (use $10 rubber connector), Pilot Jet 60, Main Jet 190, Needle Jet N-8. slide cutaway 2, Needle 5E75. The inner diameter of the connecting hose needs to be 26mm for soft rubber or 28mm for hard rubber.

Click here to find out how to adjust a carburetor for perfect running thru all the throttle positions.

Here are my recommended carb sizes which comes from my carb sizing calculator which takes into account engine compression, cylinder bore, intake type, engine size, RPM. If your engine compression is more than 130psi then you can use a slightly larger carb.