can anyone shed any light on the location of the voltage regulator for the dynamo/ generator on a vintage speedster?
It may be possible that your Vintage Speedster engine is using a generator, but an alternator is much more likely. Generators became obsolete in the 1960's, when diodes became plentiful and inexpensive. If it is an alternator, an internal regulator is also likely.
Maybe you can describe or photograph the gen/dynamo, but it's an unlikely addition to an engine, since modern alternators are cheap and efficient.
VW stuck with the old 30 amp generator til early '70s ('73-74) - they were simple and could easily be rebuilt - new bearings and brushes and good for another 40k miles. If you sill have the generator, it should be easy to track the wiring to the external regulator. It might be in clear site or hidden behing the cooling shroud. Here's picture of it and the wiring for it. If it's an alternator post the number off it and look up wheter it has and internal voltage regulator or an external one.