How do I toggle the engine on and off? it says W or S in the manual but there is no key yo change it in the in-game configuration. need it for one mission

Press 's' twice to turn the engine off. Press 'w' once to turn it on again. If you have a booster, pressing 'w' twice when the 'Boost' indicator is lit activates it (or at least most of the time it does; sometimes it doesn't work for me, usually when I'm getting hammered).