Can you crew a Vehicle that is already a creature?
You can activate a Crew ability multiple times. Normally doing so doesn't do anything (other than tapping the creature used to crew the vehicle), however since the Motorist has an triggered ability that triggers on crewing a vehicle the trigger will still occur.

From the Gatherer rulings on every vehicle:


  You may activate a crew ability of a Vehicle even if it’s already an artifact creature. Doing so has no effect on the Vehicle. It doesn’t change its power and toughness.