Ancient history maybe, but Liquid now has
SM lower headlight mounts in quantity. These are different from the EX/EXR, pics below (MXR on L, SM on RH top, EXR RH bottom) & in eBay listing:
The SM and EXR brackets look nearly the same to me except for the fender mounting 'ears' in the middle. If the
MXR bracket (which is basically just the ears, fender locator stubs, and crossbar etc.) could be piggybacked on top of the SM bracket - I checked the screw-hole spacing, looks identical - it seems like it might work for the dirt bikes. The fender holes might need 2mm or so of elongation to the front. Hopefully one missing part of the EXR-conversion kit available again.