John:  If you could find a diode number that would be great.  Essentially you are looking for current to pass one way but not the other?  Does it have to be a specific diode or could you use one from early alternator diode bridge for instance early GM with 6 diodes in them?  I have some of them around yet.  Or is voltage a factor since original would have been 6 Volt Positive ground?  Thanks, Terry