I bought a red 6v stop/tail light from Classic Leds. I don't know if they will fit into a fag end light though. They are not polarity sensitive, so can be used for positive or negative earth bikes.
I also have a 6v pilot and 6v dip/full beam on the front (I got the reflector re-silvered). Lots of light, but not great on the focus. Got those from Classic Dynamo and Regulator Conversions. The dip/full beam is polarity sensitive.
I was thinking of converting to 12V negative earth electrics, as the bulbs are more readily available. However, my newish regulator is positive earth, so couldn't be bothered to change.
Big benefit is that although they are not great, they draw virtually no power.
