I have 5303 rig to a 20 mm fork with torque arms that i made from 6 mm steel bar which is all that will fit between the legs and motor axle. i have been using this set up for few months now and have had no troubles.
yes, 1/2" bolt keeps the bar nailed to the fork and strap on top to prevent any swing. i dont see any other way to attach the motor without destroying the fork.