I was about to buy a new Yuasa B38 6v 13 Ah lead acid battery, but instead found that by performing about 5 cycles of charging during the day followed by discharging at night, the battery was restored to over 6v. Firstly I topped up the acid to the full levels with distilled water from my dehumidifier. Then I charged using a trickle charger from morning to just before bedtime. Then I connected the battery to the bike and left the parking lights (tungsten filament bulbs) on until the morning. That was the first cycle. After repeated cycles, the battery was able to retain the normal voltage of about 6.5v.
Also, I found that wrapping the semi transparent battery in black gaffer tape, leaving a window for the acid levels, looked fine.
One other thing; when reconnecting the terminals from the loom and replacing the battery in position on it’s carrier, be very careful to prevent the chromed strap from accidentally touching the positive terminal which is very close indeed!