out of curiosirt, how did you manage to break the outer casing? Usually when a motor fails the armature dies.
I am not an expert on the issue, but heard someone on these forums who is an expert on motors say that disassembling/reassembling the CIM motors without using keepers will make them loose a nontrivial amount of power.
you could buy a new one from IFI for $28.
http://www.ifirobotics.com/first-store.shtml