You can get a voltage regulator for that 100A Denso at eEuroparts.com
These guys are great and sell quality parts at fair prices.

Back to your problem.
The 940 series have a common problem (on the two that I've owned anyway) with faulty solder connections in the instrument panel.
The excitation voltage for the alternator is supplied thru the charge light in the instrument panel. If you have a faulty connection, the alternator will not receive excitation voltage, and will not charge.

This is simple to check.

Does the charge light come on when you turn the ignition switch to "on" position? Does it go off when you start the engine?
It should, and there should be 12V present at the (lone) small (blue I think) wire that plugs into the back of the alternator.
Unplug this wire and check it with a volt meter (without the engine running).
There should be (battery) voltage (to ground) there with the ignition switch turned on.

If there is, and the alternator charges, but is not supplying enough power, the problem could be in the alternator, voltage regulator, or could be as simple as a loose alternator belt.

You say that you have 13.4 Volts with the engine running, and 12.4V at the battery terminals. How are you checking this?
What is the battery voltage with the switch turned off? No load on the battery?

Start the engine and check the voltage at the battery terminals with everything turned on....lights, heater blower, AC, etc.
If the voltage at the battery terminals drops below the 12.4 battery voltage, the alternator is not supplying enough current, and you are draining the battery.
As long as the alternator can supply enough current to keep the battery voltage above 12.4V (or battery voltage whatever it is) with maximum load applied, the alternator is good.

Hope this helps
steve