You will need motors like these:http://www.ebay.com/itm/DC3V-12V-680mA-Motor-20160RPM-High-Speed-Large-Torque-Motor-for-Super-Model-/322225736876?hash=item4b06268cac:g:ppkAAOSwgZ1XreZB
I found them in a local hardware store, and for a previous project a disassembled some from a photo printer.
If they are less then 30mm, you will need to fill the space with hot glue. If you can find them exactly 30mm, they can be press fitted.
Also, If you print the flywheels, make sure to use 5 or more perimeters. There is a hole in the middle of 1mm, but that needs to be wider and therefore you need to drill in it. Make it a little less wide then it should be. Then, press fit the motor to the flywheel. (in between a vice or something similar).