As RDfella says - it's a 2 min job to get the original polarity back, costs nothing and job done.
Which does work Glen, ref your latest concern - it is, truly and honestly cross my heart etc etc, quite unnecessary to take the thing to bits to change it. Dan is, on this point, misinformed, like Bogart in Casablanca!
Polarity reversal may not suit though, if intending to run modern electrical accessories that only work on negative earth . . .
. . . in which case there are several decent regulators that would do the job. They mostly work at either 6v or 12v depending on preference - there's nothing inferior about that, if it's a concern, and most of us would count it an advantage.
The most popular one probably among folk here is the DVR2 from Dynamo Regulators whom you mention. Tried and tested to the nth degree by dozens of people here, I personally have 3, others may have whole shedsful!
The JG range might include a '12v only' unit (it used to, but not sure now). But, if you went that way you'd need to rewire your field coil connections to go between D and F, not D and E, and verify you've got the direction of rotation right when you've done it. That's because JGs were designed for Miller systems originally.
It may be worth adding that new finer-wound '12v' armatures and field windings are actually easily available (and reasonably affordable) over the counter, as are brand new standard-wound items.
