You're right in that they have not shown a center datum line. It's not the end of the world though. The center datum like is just a reference and it isn't used for anything but convienience. It's the angle the motor is mounted to in reference to the wing itself that matters. And you've got enough on the drawing they sentyou to find that out. Measure the relativeangle from the wing section chord line (the very nose of the leading edge to the middle of the trailing edge) to a thrust line axis drawn through the motor. And then set up the motor to match that angle.