My opinion:If I'm not mistaken the XR doesn't have a battery. Therefore the light emitted depends directly on the power the alternator/generator is generating, which in turn depends on the engine speed. Therefore low engine speed (when riding slowly for example) = low light.
I think Baja designs in the US (they have a website but I can't remember the URL) supply uprated (higher output) alternators for XRs.
Alternatively have a look in the back of Trials and Motocross News or Trail Bike Magazines for suppliers/electrical specialists who might help you out.