Having open armature windings is not as bad as having shorted windings... but loosing any is really going to hurt the torque of the motor...
You can use an ohm meter to check on the commutator every pair of segments opposite from each other should show continuity... and should not be making electrical contact to their adjoining segments...
Like reddragon said, it is not practical to have a small armature rewound, unless you have an electric motor re builder in your area.... :smile:KennyV