Look at it this way, a human on a bicycle exerts roughly 1/2 horsepower. Bicycle chain can certainly handle this.
Now, put a 5, 6.5, 13, 18 HP motor on there. The #40 ONLY indicates the distance between the rollers, not if it is for karts, it is not a gauge of strength, it is a measurement. It comes in different widths to suit your needs. The number can be used to identify pitch in the following way: the first digits identify the pitch in 1/8" increments, and the last digit is 0 for standard proportions, 1 for lightweight chain, and 5 for rollerless bushed chain. A #41 sprocket is therefore for 1/2"-pitch lightweight chain, a #120 sprocket is for 1-1/2" pitch chain, etc.