I had a Honda ATC that required a 6 volt 25 watt headlamp. I couldn't find one anywhere but I had a 12 volt 100 watt headlamp globe. I worked out that it would consume 25 watts when powered by 6 volts but would it be a dull yellow glow? It worked very well with a nice bright white light.
6V VW cars persisted quite a long time - some smaller specialist VW parts retailers still carry 6V bulbs.
You can get 6V H3 bulbs for high power rechargeable hand lamps - but they're single filament, so you have to replace the headlamp unit with 2 separate ones.
Honda motorcycles used their own unique bulb fitting - if you can find a 6V H4 bulb, you may be able to get the reflector it fits from a Cibie kit.
About the late 80s - many motorcycle makes started using H4, they're hard to find - but I'm sure there's such thing as a 6V H4.
If its AC lighting - you can't change the bulb wattage much. More and more automotive LED bulbs are appearing on the market. The lower power consumption leaves room for rectifier/regulator etc to make it work with the supply you've got.
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