A 6v harness is a heavier gauge wire and would handle 12v with no problems but if you don't care about original appearance you can get a modern harness much cheaper. I've not used a modern harness so I can't say which are easier than others. Someone will chime in but in the meantime you might do a search of the forum. I remember posts in the past dealing with just that subject.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.