I got a great deal on these 2 Zodiacs and 54 other watches on an online auction, the men's watch appears to be a 723-921B, with a 72B movement that is "not running". The crystal is cracked and the back cover is gone.

I'm thinking about restoring this watch, is it too far gone? Does anyone know what crystal will fit it? There seem to be case backs for this model available online.

Also, how should I go about getting the movement serviced?

That looks to be in very good condition. I can offer you this on the service question.

https://www.vintagezodiacs.com/watchrepair.php

Thank you,
It's still in the mail. I'll email you when it comes in. I plan on having BWG service it if all goes well. 

Also is 723-921B the correct ref # for this watch? I can only find one watch like this one for sale anywhere in the world. There must be very few of these left.

I am unsure of the case number. Yuri, is this something you can help with?

As far as rarity, just evedently not a popular model back then. I have this one.

Yes, 921 is a correct ref number for this model.
And you have 72B caliber. So B suffix is correct too.
Although Your watch has stainless case. So third digit must be 2. Not 3.
Right ref # for this particular piece must be 722 921B

Thank you Yuri, you are so much better at that than I.