Estimate small engine horsepower by dividing the engine's cubic centimeter measurement by 32.5. There is no direct conversion between cubic centimeters and horsepower because compression, efficiency and engine health also affect horsepower. This estimation yields the horsepower at the motor output shaft, not necessarily at the final drive.
Most engines have a horsepower rating listed in the documentation for that engine. Consult the manufacturer or owner's manual for an accurate horsepower rating. Engines usually have a model, serial or identification number carved into a visible flat spot on them. This can be referred to when looking up engine specification data.