Joe: I have a few of the open rocker ENYA engines (35 and 40).
Although a bit heavy, they are remarkably smooth and quiet, also very easy starting.
I would not hesitate to run the 40 on any 40-size plane - you will just have to shorten the nose a bit.
The Brodak/Johnson P-40 would be the perfect candidate!
I run mine on Brodak 10/23 or Powermaster 10/22.
Before you run the engine, pump some oil into the crankcase and tilt the engine around to insure that all internal parts are lubricated. The excess will blow out through the breather.
Some oil on the rockers can't hurt.
These engines turn at a moderate RPM but develop substantial torque. 11-6 and 11-8 props work well on the 40.
Bob Z.
ps - SAITO also made a neat little open rocker 30!