Phil...To change your BN from a 12 volt negative ground system back to a 6 volt positive ground system you'll need to change:
12 volt battery to 6 volt...12 volt alternator to 6 volt generator & regulator...12 volt starter to 6 volt starter...12 volt coil to 6 volt coil...12 volt light lamps to 6 volt lamps...You're virtually swapping out most of the electrical system
In other words, there is an expense involved in order for you to change it back to its original 6 volt system. If you're not a purest, or don't care about the "correct police"...it'll be a lot cheaper to leave it just the way you've got ir now