The bike may be 6V. I don't know British bikes, but I would double-check.

The headlight is also probably sealed beam as well... as a giant light bulb. Most suggestions here will not work.

I would buy a new cheapo headlight off ebay that takes a separate bulb. Then use any bulb and dimmer you see fit.

I would be tempted to get it running. If nothing else, I'm sure putting around a parking lot with it would be fun.