EVguy--

I think the difference between a 12v alternator and a 24v alternator is additional turns on the windings and the use of a 24v regulator instead of a 12v regulator. The extra windings will allow the alternator to produce more voltage than you will need at 12v but should not prevent it from working OK at 12v if you use a 12v regulator.

Perhaps you could scrounge up a 12v regulator and install it yourself or you could take the unit to a repair shop to see if they think this would work and if they can do it.

---Caveat: I have never tried this myself.

Or, perhaps a better idea is to look for an alternator from
an NPR (if NPR's are 12v?) or a C223 as both of these have an Isuzu type vacuum pump on them.

Paul