If you are using a bridge wire between the F and D terminals and putting your multimeter between that wire and the body of the dynamo you should be getting easily 10 volts on the meter when you rev the engine a bit (no need to go mad on the revving) ...changing the multimeter leads over will show if the dynamo is producing pos or neg earth. I'm assuming your multi meter is operating correctly?
Basing the output on a bulb doesn't really tell you much...if its a 6 volt bulb it may be bright but your output may still be under par.
You really need to be sure that the dynamo is putting out good voltage before deciding on a regulator but I would recommend the DVR2 regulatorhttps://dynamoregulators.com/dvr2.php
You need to decide on what polarity you want before you buy but a DVR2 can give you 6V or 12V.
The problem with running a 6V dynamo on a 12V system is that, instead of the 6v dynamo being able to balance charge on a 6V system at say 30 mph you need to be doing 40mph average to keep a 12v system fully charged. Do you really need a 12v system?
As I said before...work your way through the procedures set out in the first post in the electrical section.