Well, the stock regulator is 40 years old and it wasn't very good even when brand new, so the obvious answer is to go with the "finned" regulator.
However, not all "finned" regulators are created equal. Generally, if it is finned, that would indicate that it is a combined regulator / rectifier and it would replace both on your bike. The more complicating factor is that regulators come in two distinct flavors. Those that provide a regulated battery voltage to the rotor and those that provide a regulated ground to the rotor.
Your 1971 XS650 requires a regulated battery supply to the rotor. Most "finned" regulators provide a regulated ground, so you will have to find out more about the regulator that you have. If it is of the regulated ground variety, then you will have to do the nylon screw mod to your brush holder.
As far as wiring is concerned, your best bet would be to go to MikesXS and buy the appropriate matching plugs for your regulator and rectifier and then modify the wiring on the finned regulator to make it plug and play without having to cut up your factory wiring. Unless, of course, your finned regulator is the model from MikesXS that is designed for your bike and you just didn't want to tell us that!
