First, its probably a good idea to clean the crack first with some alcohol, just in case there is any lubricant leftover on there as it will prevent the glue from adhering well.

Next get a pair of needle nose pliers(preferably ones with grooved "teeth") Then wrap a rubber band around them so they hold tension. The tension should be something you can fight against, but not so tight it may crack the gear.

Fill in the crack with a little bit of super glue. Then use the pliers to push the crack in the gear back together slightly.

Necessary NOTE: Use a thicker type of super glue. I recommend Loctite gel type. If its too thin it will get in the motor bearing, then you are SOL. Also a little bit goes along way. I used a dental pick to work it into the crack.

I also wanna reiterate NOT to get glue on the Bottom of the gear or onto the main part of the motor. IF you do, immediately remove it before it dries. There is super glue remover available, but I find if a little gets in the way, just turn the shaft to break it free and clean it with an Exacto knife.