Did Yanmar put the same Hitachi alternator and regulator on the JD 750 as they used for their own tractors? Yanmar used the Hitachi alternator and regulator specified for 70-73 Datsun (Nissan) on their own mid and late 70's models, up into early 80's for models that continued unchanged.
You might take your alt and regulator into Autozone to put on their tester and explore if a $19 aftermarket regulator specified for '72 Datsun happens to be the same. Or if your stuff is Hitachi it might match Japanese car stuff of the year that the 750 was first produced. (1978??)
If this works (or even if it doesn't) - stop by the Yanmar forum and tell us what you found!