```
Horsepower = (Displacement in cc / 1500) * Efficiency
```
Assuming a typical engine efficiency of 25%, we can calculate the horsepower for a 420cc engine:
```
Horsepower = (420 / 1500) * 0.25 = 0.07 * 0.25 = 0.0175
```
Therefore, the equivalent horsepower for a 420cc engine is approximately 0.0175 horsepower.