TnA Gordon Problem Resolved.

[Super]

I agree with Mr Ripley, Glue is always only a temporary fix and using Epoxy might ruin any chance for a permanent repair later. You can never get those gears back to the same tightness around the axle that an uncracked gear has so no matter what glue you use it will still be prone to spinning on the axle under a load eventually even if the glued gear remains glued. I recommend just what your plans are, start out small and disassemble engines, especially if you have junkers, until you feel comfortable opening one up and either replacing the cracked gear with a new one or the whole axle. Then the next step would be disassembling the motor itself to see what makes it tick to learn how to repair. Your best tool for learning this is your camer or phone where you can take a picture of each step of disassembly so you can refer to the pictures upon re-assembly.