From Honda Wiki
Honda's NSR 50 is one of the most commonly raced bikes in Minibike club road racing. Though the NSR 50 is not street legal in the United States, safe and legal racing environments are available in many states, often found at kart racing tracks.
The NSR50 has a 49cc water-cooled, two-stroke engine, a manual clutch with six gears, and typically falls into class with other two-stroke 50cc road bikes such as the YSR50. Though, it is also commonly classed with up to 110cc four-stroke bikes.
The front sprocket is identical to that of the more common Honda xr50 dirt bike.