Before you go to a lot of possibly unnecessary trouble - don't give up!
Keep on with the WD40 soaking and rather than trying to "jerk" things
loose by running the thruster or banging the electric motor, try this.
Put a strap wrench around the electic motor, either a chain wrench or a
rubber strap one. Then put a lengh of pipe over the strap wrench
handle for added leverage. Then carefully torque this up, trying to
turn the motor just a few degrees either right or left. Do this with
the thruster retracted: the lower housing will act to resist your
turning of the motor. Be patient, turn it right, then left, then right,
etc. Be patient and sooner or later it will break free just a bit -
then keep nudging it back and forth. At first you may only be able to
go a few degrees at a time, but it will break loose after a while.
Don't give up!