On my older dirt bikes and Hondas ('70s CB and CL
series), there appeared to be no regulator at all but
the battery. Perhaps you have shorted cells that
account for the low voltage.
I would suspect no readily servicable parts inside a
regulator assembly, but I'd look for other causes of
low charging voltage before chasing the regulator.
In several cases I simply replaced the battery with a
large electrolytic capacitor to get the bike to run, I
was a "poor college student" at the time with plenty
of access to electronics surplus but very little cash.
Those bikes had no electric start anyway and I saw
no use for the battery (lights work in either case,
though perhaps burn out quicker at sustained high
speed or get dim at idle).
I bet anything after the '80s probably does have a
regulator. But regulators only have so much authority
against limiter alternator current and shorted battery cells.
Measuring the charging current, rather than voltage,
might be illuminating (heh).