Since it's on the axle, I think you're really going to have to find the exact part.
If you could find the exact same size gear, you could pull the axle apart and press the gear on and then put the wheel back on.
I replaced a gear like this on a CC II Shark a few years back. I used a vice, a punch, and some different sized sockets to do the pressing off/on.
I marked the wheel and axle position before removing them so I could line up the splines when reinstalling them.
Worked pretty well.