The ES stator tries to get 10 pounds of manure into a 5 pound bag. They rewind a stator core with as much wire as they can cram on. Actually winding the wire on is a bit of an art and is a significant difference between stator companies and directly affects the amount of power generated. The stator wires are carrying -- well -- 100 watts more than the stock windings. With no cooling space between the adjacent wires that are crammed on the stator lobes plus all the extra power, the windings overheat. The heating tends to be concentrated where the stator wires and the harness wires are bonded. In an ugly twist of fate, on the FJR this happens at the top of the housing where there is almost no oil spray to cool things down and the top is where rising heat goes. Over time the heat and the expansion/contraction of the wires compromises the insulation on the wires. When the insulation between two adjacent lobes breaks down electrical current begins to leak. Sometimes it is the heat working at the stator wire/harness wire junction where the wire breaks down and starts to become a resistor instead of a conductor.
A rewound stator is only part of the voltage generator. What isn't being changed is the rotor. The stock rotor with what ever magnetic poles the factory used remains unchanged which forces all the power gains to me made through the stator. Oh but to have a rare earth rotor that makes much stronger magnetic fields to really excite the stator
The Gen II is designed as a system. The rotor on the Gen II is longer than the Gen I and the stator is larger too. This lets the whole system work at a more optimum design so that cooling isn't a problem. You can indeed add a Gen II charging system to the Gen I. It requires the longer stator cover, new rotor (getting the tapered rotor off the crank is a first class *****), new stator and new voltage regulator. Oh ya, the R/R on the Gen I is up top near the front of the frame, the Gen II R/R is down by the lower back of the engine and within sight of the rebound damper adjuster on the shock. Some creative wiring would be required.
It is possible to get a Gen I stator wound to produce the extra 100 watts and remain reliable. You wouldn't want to pay for it. Someplace I have written a short thingie about wire coatings, insulating layers and potting stators for long life. If you are having trouble sleeping I will see if I can find it.
And, as a side note, the Gen II's bigger rotor adds flywheel weight. Give 'n Take -- Take: the larger mass will slow down the engine's ability to rev as quick. Give: The added mass will help smooth the engine and smooth the idle. Fortunately, the scale is small because the weight increase is small.