There are several solutions available to address your problem, and I have done them myself for others.
The first solution is expensive, but looks good:
First, get one of these, a B&M Nylon Linear Pull Brake Mount...
Then, get one of these, a Schmidt handlebar band-mount for oversized handlebars...
Invert the latter in the former, and insert a sawed-off section of PVC pipe into the handlebar clamp on the Schmidt mount. Attach your light (originally intended for handlebar mounting) to the pipe. Done.
Other, DIY solutions can look a bit less professional, but are as functional:
These work in a similar fashion using hose (jubilee) clamps, PVC pipe and a stock crown-mount light bracket. I have made a similar arrangement for someone using a Minoura Besso fork-mount bracket attached on the to nuts stacked inside the opening of a standard crown bracket. The Minoura Besso ( http://www.sjscycles.co.uk/minoura-besso-lh-50-bracket-prod22461/
) ends up mounting the side of the crown bracket and the light mounts to the Besso as if it was a handlebar. For something as light as a 'bar-mount headlight, PVC and some solvent cement are a match made in heaven for relocating the light elsewhere.
The main thing to keep in mind is try to keep the cylindrical light mount as close to the fork crown as possible and still allow for clearance. This way you avoid unnecessary stress on the brackets and the light will be as stable as possible.
Hope this helps.