The voltage regulator you mention is actually just a "cut out relay" as used on all three brush generators.
It simply closes when gen voltage increases above battery voltage and opens when it drops below battery voltage. The electrical current is regulated by the generator Third Brush.
A Ford Model 'A' relay will work perfectly for the three brush generators until you can find the original unit.
Do You own a car built by Durant? 1923 Star Touring, 1927 Star Coupe
Don is correct abouth the Ford units. If you use one of the diode versions, be sure to use the T one because it is negative ground.
If you want a better and more original looking one (with out the Ford script) and square shaped as the originals. Most tractor stores sell a cut out that was used on farm tractors up through the sixties, these are readily available and not expensive. Most Tractor Supply Stores have these in stock. Just tell them it is for an Allis Chalmers C , not a Durant or Star !!
Hope this helps....Frank ---
Do You own a car built by Durant? Now 13 Durant & Star cars & a Durant Dort buggy