Is this just because 6V systems are all very old, and they are having to fight old, oxidized connections, and having to push twice the current through (v. a 12V system) to get the job done?
I can't think of any reason why a 6V system WITH CLEAN PROPER CONNECTIONS should be "worse" than a 12V system.
Matter of fact, wouldn't that imply that a military-style 24V or 28V system would be even better for a bike?
Enquiring minds want to know.
Good on yer, getting that old nail back on the road. Sounds like fun.