I would like to suggest a compact jet turbine engine. Something alike to what is used in commercial jet powered helicopters and small airplanes.

I imagine a part that would have only fuel in and power out and be quite compact, 2x4 or 2x5? Don't know if parts of such dimensions are even considered (most parts are 5x5, 3x3 or 1x1)

Pros:

Compact size

Simple operation (like jet engine, no cooling, intake and exhaust pipes)

More realistic for small helis/planes

Cons:

Uses jet fuel - is more expensive

No thrust provided at exhaust

Has slow throttle response