I had a look at the microjet engine. With 65 Lb of thrust, and with 36
cm long, 16 cm diameter, a 400 Kg plane should be able to climb at 1,3
m/s sustained (25 Lb for the aero drag, rest for climbing). Going to
1000 m would take just 12 mins and 11 Kg of fuel. I ignore the cost of
fuel, but at 1 $ /kg, that makes for cheap tows to offset the cost of
the engine.

Next, could the exhaust temperature be solved by embedding the tiny
engine behind the pilot and letting the exhaust out throug some
openning in the aft taper of the fuselage, and well below the tail
planes. This would however need some adjustable intake port in the
fuselage. But in all cases it should all be much simpler that folding
out a propeller with or without a piston engine.... .