RE: OS 61 Heli - Aircraft ?
The crankshaft on the heli engine is likely shorter than one on an airplane engine. You may have difficulty getting a prop attached, let alone a prop and spinner. There isn't a different cylinder head you can attach, the heli engine is not designed to accept the airplane cylinder head. If you run a heli engine in an airplane, you may need to restrict airflow through the engine to prevent overcooling. Other than that, you can use the engine wherever you desire.