I saw a clever motorglider a few years ago that had an engine on a
retractable mast - above and behind the pilot (it was a one-place).
The unique feature was that it had a one-blade propeller (there was a
counterweight on the other side), and some means of positioning the
prop after shutting down (so that the engine and prop could fold down
completely inside the fuselage). It was more than a sustainer engine-
I watched the aircraft take off and climb out unaided. Don't know the
type, but have several photos of it somewhere.

Dave