Heres my problem with adding the quad rear. Your making the buggy to long for it's width. Your putting alot of stress to the center of the frame. That rear shock is meant for much less weight, let alone the added torque from the length.

I would mount the motor in a cage. then use the front arms on the rear of the buggy. Then do as Greg did for a cheap irs.

Or I would make a cage for the motor with room for a swing arm. Making sure that the sproket on the motor lines up with the pivot. I would place a sproket there for the drive chain to run back to the axle.