Engines are not the only way to generate power. Reactors can provide power but do not automatically generate power during the power generation phase.
Power generation from a reactor is done the same as pumping an engine (ENGvs 8 to generate one point of energy, -3 per each extra point.)
When a ship with a reactor explodes, add one for each reactor to the size for explosion damage. Due to the hazardous nature of reactors, they are at +2 for damage when hit.
Reactors do not need external facing.