I like the idea of a practical reg/rec test.
If the 6V stator makes about 20VAC open circuit when you rev it a bit you are probably there. The 12V stators hit about 30V, but you should just need (1.41 x 14) to hit top-up numbers. Worst case you already have a 12V reg/rec in the parts drawer.
You'll need to bootleg in a 12V battery for the test.
Fused 10A jumpers & #16 wire or better from the 12V reg/rec to a handy car battery will get you by for a test. If it works, it'll be pushing 13.5 V or more back to the battery at 1500 RPM or so, THEN you have the guts of your new system for the price of a reg/rec. Good test for that KBPC 6/12v regulator!
Just put the stator AC to the 12V reg/rec and run the 12V battery jumpers to the test rig car battery. Let the 6V (+) ground battery run the ignition normally for the test.
There's no real difference between 6V & 12V for insulation purposes, wiring is usually rated 60V to 300V anyway, way beyond what it'll ever see. 12V Load currents will be 1/2 of their 6V counterparts.
The ammeter just knows current. IF the OEM 6V reg/rec puts out 60W, the 12V version will be half that, so the ammeter will just deflect less.
As Stinkwheel says, you'll need to reverse the polarity IF you go negative ground.
Test it - you already have a reg/rec in hand, right? If it will charge a 12V battery at 13.5V - 14V when above idle, that's good enough. Then build your (-) ground system.