From OS website FAQ.

Can I use a helicopter (-H) engine in an airplane? Why or why not? What muffler should I use?

The large cooling head on heli engines often provides too much cooling for the high-airflow application of an aircraft. Occasionally in a tightly cowled scale design a heli engine may be preferable to resolve cooling concerns. If the engine is produced in both forms (heli and aircraft), then the aircraft exhaust can be used on the heli engine and vice versa. Under no circumstances should an aircraft engine be installed in a heli application without a heli head or other extensive cooling precautions taken.