I made a homemade spinning wing back when the motorized mojos were just being introduced (mid 90's?). Back then the mojos sold for over $200. I used a yard mallard that I cut the base off, made homemade feet out of a surf laundry detergent jug, legs out of pneumatic air hose, a remote controled toy truck motor and it's transmitter, made the mount out of an old golf cart handle so I could change the angle of landing, mounted the motor with a piece of thin sheet metal, wings out of plastic cutting boards, attached the wings to a piece of pvc and had a great, inexpensive motorized spinning wing decoy. After about a year, when mojo started selling their motors, I substituted the truck motor for the mojo motor and still use the spinner.
So that's some suggestions......however, it would probably be easier and cheaper, not as much fun though, to just buy the motor. Your biggest problem would be in the mounting of the truck motor.
Here's some pics of the old boy and how mine was rigged. (with the new mojo motor substituted) More pics on photobucket:http://s766.photobucket.com/user/tenfingergrip/library/Homemade%20Spinner