Ed,
I am buiding my second yellow monocoupe. Dumb thumbed the first one. The first one had a Magnum 91 FS. It flew ok, but needed long take offs without flaps and didn't climb with much authority. My second one will have an OS 120 FS or Saito 120 . I have both engines, just haven't made up my mind. Should have plenty of power. I have read where some guys flew it with a Saito 100 with no problems. One guy in our club had the red one and flew it with a Saito 170 radial. Most of these I have seen flown with gas.
Sam