Update - Thanks to everyone and their suggestions. Turns out I didn't have to pull that rear bearing housing at all. I forgot the generator spins, and after remembering, I was able to bump it over and access the diodes.
Spoke to the sole Mecc Alte dealer in the US. They sell a ton of these diodes and are 2 weeks back-ordered.....at $37.70 EACH. He was helpful, but didn't have any specs on the diodes and said the only place that sells them is Mecc Alte.
I pulled one of the old diodes off and there's zero markings, bands, nothing. I googled Mecc ALte diode and found these which are visually similar:
Diode D2-25A Mecc Alte Alternators: ECO/ECP 28/31/32
www.gensetcomponents.com
They're in the UK, but I used the 'D2-25A' portion and found these on Amazon which are 20 for $7:
They are visually similar, good to 1000v, and listed at 20A. I was unable to find any 25A, but I was willing to risk the $7 investment.
Just got done cutting the existing diode leads as close to the bad diode as possible and soldering in 6 new diodes (diode bands all face the same way - on the leading edge of the diode if it was to spin clockwise when viewed from the end of the alternator where you're working).
Fired it up and for the first time the control panel lit up! I'm now getting 56vac line to line, and 32vac line to nuetral. Obviously not where we need to be, but I'm making progress. I suspect I will have to go back to step 1 of this thread and flash it, but I ran out of time and had to call it a night.
I'll be out of pocket 2 weeks but will update you all when I'm able to work on it again.
Thanks again for the help and if you've got any ideas why the voltage is so low, let me know!