I had a bit of interest in the YZ-125 powered go-kart so here’s a bit more information about selecting a gear ratio.
Here is a basic example of a kart I used to race. The tyres diameter & circumference will vary a little bit with tyre pressure and weight in the kart but give a good idea for the initial setup. With a 9-tooth sprocket on the engine and an 81-tooth sprocket on the axle gave a 9:1 reduction from the engines 18500rpm (I miss labeled this peak power, which happens earlier, but this was the peak RPM I would aim for on the tachometer recall) to ~2056rpm at the axle. The estimated top speed of 123kph approximately matches what was seen when racing.
Now the YZ 125 has a six speed gearbox and a much broader range of speeds, depending on the final ratio from the engines output shaft to the axle. The six gears are a relatively close ratio to keep the motorcycle (as normally installed) in the narrow power-band of the 125cc engine.
It is a good idea to keep the ratio with a realistic top speed, as in a real life situation drag will increase exponentially and you may not reach the predicted top speed. A shorter final ratio will be easier on the clutch too.
This graph shows how the gears will overlap. In some situations it may be better to gear the kart up or down, shifting this spread of speed vs rpm, to reduce the number of times you have to change gears or to keep the engine in the optimum rev range for the corner exits.