3-volts severely limits what you can do. It's probably best to just keep your vintage light just as it is. An LED conversion is a possibility but the reflector in your light is not going to provide a good pattern so your not going to be happy with that. High Power LED's require heat sinks and that complicates the conversion as well. It's best to build from scratch. Here is a good site with lots of information on building very good lights for your bike.
http://pilom.com/BicycleElectronics/...lectronics.htm

Here is a good place to obtain high power LED's and optics.
http://www.dealextreme.com/