On the old farm tractors and American made vehicles there were two lugs on the generator body. One is the main power output from the armature and the second one is the field coil.
They are either set up in a field to case or a field to armature configuration. That is in the field to case set up if you connect the field and armature lugs together to get its full output (power to field lug). In the other design you tie the field lug to the case to get the full output (ground field lug).
One other problem is that back then they did not have diodes so they used a voltage sensing relay that would connect the armature to the battery if the armature voltage was high enough and then disconnect it when it dropped below a specific voltage That way the battery would not feed back to the generator if its output voltage was to low.
On the few solid state ones I built I just used a 20 amp dual diode type rectifier in a TO-247 case to replace the relay. Given that the old tractors I was working with only had about 15 -20 amps peak output just using the metal case of the old regulator for a heat sink was good enough.
I am not sure if current regulation is absolutely necessary. Many of the American made mechanical regulators didn't have it built into them. If the regulator is a square one it typically only has the isolation relay and the voltage regulator relay. Some had a current bias winding over the voltage control relay and used a sort of weak current bias to drop the output voltage if a high enough current was being drawn. The longer rectangular ones are typically the type with the adjustable current limiting built in.
Although many old charging systems didn't even have actual voltage regulators. Rather they just had the isolation relay and a specific value resistor inside the regulator box that just kept the generator at a proportional current output . The faster the generator turned the more amps it delivered. It was crude but still millions of them were used.
You are in fact right that they used a rather crude mechanical PWM of sorts. Thats probably why a basic comparator design works rather well. It uses the same basic effect as the mechanical system but is much faster and more accurate plus uses actual diodes and provids far better spike suppression than the buzzing contact regulation systems ever had.
Just some things to think about.