Repairing a crown with missing enamel: Possible just to repair the enamel instead of replacing the crown?

I have a 45 year old crown on an upper tooth. I think it is #28 on a dental chart. It has recently lost a piece of the enamel that is visible from the front. It was well done it appears, since it has survived so long and continues to be really stable. Is it possible to just have the enamel repaired? My insurance does not cover crowns and since the work seems to be very good I am reluctant to have the whole crown replaced.