Actually, if you use a replacement (aftermarket) solid state regulator, it COULD work just as well as an isolated field (they are NOT single or dual field)
The real difference between the older "round back" and the newer "square back" is that their low RPM output is a little better.
What I mean is, if you take a late model squareback, and ground one of the fields, and then use one of the regulators posted above, it should work just as well as the 70/ later regulator.