The best version of the UFO engine known to me is done like this - you need to take a Hower Block, place 2 Steering Hinges above it, to which you need to attach a Shield 2x1.
To activate the engine, the Shield must move away from the Hower Block.
Steering Hinges should be configured like this - speed=2, Angle=5, Steering Help=0, Strength=100.
The power of this engine is approximately equal to 875.
To stabilize the movement, you can make 2 identical engines and put them together in a mirror image.