Thanks, Ebay! Thanks, Ebayer who gave me a good deal!
Is this thing from a video game or what?
There are four large screws that hold the retaining block onto the main structure. This block holds the source and shutter motor, along with some electronics. Once that's loose, you have to unscrew the source itself from the underside, and there is also reference to "loosening and tightening the back screw ones from underneath" as opposed to tilting it and then taking it out.
You can see on the old source that indeed, behind the collimator plate, the lexan window had all but disintegrated. Behind it is the intact beryllium window.
I didn't try to replace the lexan window, as i don't plan on touching the beryllium window. I did, however, try to cover the gap up between the source and the collimator window, as presumably the window would have also protected somewhat against dust or anything else in the air stream directly pointed at it.
Source back in place, I decided to just go ahead and put all the shielding back (rather than put the case on and test it, in case something was wrong). Along the way, I left many traces that it was an unprofessional who had to shim the shielding or use twisty ties instead of zip ties. Also, no screws made it back into the same location.
Finally, there was just the case left... and I looked around and it was the evening! Everyone had gone home! And you need 2 or 3 people to lug the case around! Gosh dangit to heck!
I really want to get the source tested before I go to oklahoma for work this week...