If you can link to the part online, we might be able to help you further. Which motor are you trying to attach to?
CIM's use a 2mm key
Most AndyMark gearboxes (including your Kit of Part gearbox) use a 1/8" key.
None of the other motors have keyways. Usually the gears are pressed onto the shafts.
http://www.andymark.com/Machine-Keys-s/83.htm