A person could likely convert a model A generator to a regulated 2-brush type but they would have to add external terminals or have properly insulated wires exiting the case for connection purposes. This type of alteration would be hard to hide for the purist in order to show the car for points. It would be fine for the driver car but would require wiring modifications.
If you want to try this make sure the voltage regulator is the type that supplies voltage to the field windings and not the kind that shorts the field windings (where the voltage is supplied at the other end of the field windings).