If you locate one you may need the associated tachometer. I think it counts pulses which would depend on the alternator speed. Of course the alternator speed is governed by its pulley and you need the correct ratio between the main pulley and the alternator pulley. You may need to do a bit of homework on this one.
I feel your pain. The generator on my Ford 3550 is set up the same way. I converted it to a GM alternator and now can not run my proof meter. Which was missing when I got the machine.