WAD to move. W makes both engines fire equally, A makes the left one fire double force, and D makes the right fire double force.

I made this back before Joints were a feature, so this problem I was trying to solve is now trivial thanks to Joints. Joints allow you to attach solid objects to each other, while also allowing physical forces to affect objects, which here would allow you to add force to the engines and then have the engines push the ship.