Hello George Marsh,
Voltage output of an alternator is about 90V A/C. So you can get 90V dc if you rectify the voltage , without any step down transformer. Regulator assy will step it down and rectify it to the needed voltage. In automotive applications that voltage is 13.8 to 14.2 volts DC. The regulator is what will give you the output. I would think that if you the volt amp thing and keep the wattage the same as the original output, all will be well. Let me know if you try it.