NSI out of Arlington, WA has been building EJ series motors for airplanes for years. Truely beautiful work from what I hear. I have tried to find their website several times but have failed. Here is an description of the company. I would get in contact with them as they are the experts in this field. Good luck
NSI engines are converted from Subaru automobile engines and are built and marketed by NSI Propulsion Systems, Inc. of Arlington, Washington. NSI sells their conversions to builders of homebuilt, experimental aircraft. NSI was founded in 1993 in its present form. All NSI Subaru conversions require reduction of propeller rpm from that of engine rpm; i.e., a Propeller Speed Reduction Unit (PSRU). The NSI PSRU incorporates planetary spur gears and are of the firm's own design. Most of the information here has been compiled from the NSI Website
as well as "Kitplanes" (KP) and "Sport Aviation" (SA) magazines. The applications below are available as firewall-forward kits for the various New Glasair, Van's, and Zenith kit aircraft. All of the NSI engines are non-certificated engines for use in experimental, homebuilt aircraft.
Not sure if any of this has any relevance to helicopters but just thought I would share