Many small portable sawmills use an engine in this horsepower range. For example, the Log Master Model 16-25J uses a 25 hp engine. With a portable mill, you load in logs, and it cuts out dimensional lumber. Thousands of configurations exist, but all use an engine in the 20-to-30 horsepower range.
Many small portable backhoes use an engine in this horsepower range. These machines are towable, and usually have a 1 cubic yard bucket. While these machines do not have the capacity of larger backhoes, they are useful for trenching and light digging work. For example, the Dirtmaster backhoe works for landscaping and trenching.
This engine can power a small airboat propeller. You will have to fabricate bracketing, and fabricate a propeller bushing. While your airboat will not be as powerful as one with a V-8 automobile engine, it is small and easily towable. For shallow inland waters, this engine is ideal.
This engine can power a one or two person Go-Kart efficiently. You may be able to find a Go-Kart with a "blown" engine. If you do, you can easily adapt this engine by welding in the appropriate engine mounts. Because it produces 27 hp, this engine is not for smaller children's karts. For many karts, you will have to adapt in the clutch and related components.