I think you're all really missing the point?, voltage doublers are really only suitable for low currents, not headlight bulbs. Switchmode systems could work?, BUT probably not for this application.
It's been mentioned that 25W isn't a very bright headlight, so it's obviously off a pretty naff 'motorbike', only a moped or something similar?. These normally feed the headlight directly from the alternator, so the voltage changes with the speed of the engine, and so does the headlight brightness. The alternator itself is designed to 'self limit' it's voltage output, so once you get to a certain speed it doesn't go any higher.
These don't seem very good conditions for a switchmode converter?.
I used to have a Yamaha DT400 (400cc trail bike), this had similar crappy 6V lights direct off the engine - I thought long and hard about how to improve it, but never actually did anything about it. My thoughts amounted to this:
1) convert the entire electrical system to 12V.
2) fit a MUCH larger 12V battery in place of the puny 6V one.
3) rewind the alternator to provide a higher power 12V, without the speed limit on it.
4) build an electronic charging system to prevent overcharging of the battery.
5) feed the headlight from the battery, like sensible lights.
The sticking point really is rewinding the alternator, particularly for me, as at the time it was my sole means of transport - so I couldn't afford time to mess about with it.