6V x 85A = 510W
510W / 12V = 42.5A
I'd suggest a 45A/h battery or slightly above, anything up to 60A/h would be fine but make sure your dynamo/alternator has a decent output in A to cope with a bigger battery.
If i knew what dynamo you currently have, i could probably suggest an alternator conversionbut that would almst certainly need a negative earth conversion too.
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Cheers
Dave
Next Door to Top-Gun with a Honda CR-V & S Type Jag  Volvo gone but not forgotten........
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