BERT:
The part LM723 ( VOLTAGE REGULATOR) for the Mark8 controller is for practical purposes not longer available since it has not been made for at least 10 years.
There are other solutions using the regular alternator regulator -- though one needs to open the regulator, and some are really sealed.
There is a lead, the one that comes form the battery and the positive side of the rectifiers that go to the regulator that sometimes can be seen into the regulator feeding a resistor string, one resistor going to a second resistor ( goes to negative lead) and from this two resistors junction a connection going to a Zener or other semiconductor.
The upper resistor coming from the battery connection can be paralleled ( from batt point to junction of 2 resistors) with another resistor at least 20 times ( if it is made variable) to control the output voltage that at the same time controls the charging current.
Also, one could use a potentiometer resistor string like the one shown in the Mark8 article and the center pot lead going to the junction of the 2 resistors -- this way one does the same control.
I am working on a regulator for the 12VDC_Power group using a TL494 device to do what you want to do in a very simple manner and with the potential of been able to set the charging mode of the alternator:
Bulk, Float, Equalize, initially done manually and later with a small microprocessor.
How soon do you need the regulator ?.
Nando