The Shindengen SH847 is a promising candidate. I might try two of those to replace regulators A and B (after a lot of testing of course!) because it is a 50A, MOSFET-based three-phase series regulator-rectifier. It regulates to a sufficient voltage for LiFePO4 batteries and the output should have low enough THD that the ECU will be happy with it. That should greatly reduce both regulator heat and stator heat.
But I just noticed while I was writing this that I might have totally gotten shunt regulators wrong (so ignore that)! I started reading https://www.apriliaforum.com/forums/showthread.php?240785-Facts-about-shunt-based-regulators and I think I am starting to understand why shunt regulators are so popular despite being popularly seen as power-wasters that constantly load the generator at 100%. I thought that shorting the stator windings would be the same as pulling the maximum 600W load off of the generator. Apparently that is wrong and the power waste (and the popular belief that shunt regulators can cause stators to overheat) actually is a function of the resistance in SCRs used in the regulator. Modern MOSFET-based shunt regulators dissipate less heat so they are closer to a true short and apparently, some sort of inductance magic happens, and the PM alternator is actually almost totallymagnetically unloaded!
