Thanks to Ron Twellman, (Curator of Collections, EAA AirVenture Museum, rtwellman@eaa.org ) for these photos of the engine, described in Aerofiles as 

25hp Jacobs outboard motor modified to air cooling (now in EAA Museum); empty wt: 180. The first controlwing airplane to do any real flying.(14763)

Responding to a request for photos of the engine, Ron noted, 

"I went over to Pioneer where the engine is located and took some photos that are attached.  I've never had the chance to do any research on this engine.  It was just identified in old paperwork as the Spratt Controlwing engine.  So if these photos don't jive with what you know about this engine, please let me know and we'll try to figure it out." - RT