you sure it's 6 inches? Are there any part numbers on the light assembly you're trying to replace?
https://www.amazon.com/LIGHTING-Motorcycle-Headlight-Headlamp-Halogen/dp/B00GNU34PE/ref=sr_1_fkmr0_1?ie=UTF8&qid=1472145325&sr=8-1-fkmr0&keywords=6+inch+6v+headlight
http://www.amazon.com/LIGHTING-Motorcycle-Headlight-Headlamp-Halogen/dp/B00GNU34PE
I binged "6 volt headlight motorcycle" got lots of hits.