If the crack is visible I would try repairing the key with a few drops of Methalyne chloride. The solvent/adhesive is very thin, and it is drawn into the crack with a capillary action, therefore, just a few drops is all that is necessary to make the repair. After the glue sets up for about 48 hours, it will be just as strong, if not stronger than the key was when it was new. Take a look at: https://vimeo.com/103296536

Hope this helps,

Gary