(Welcome to APUG!)
I'm not sure I fully understand what or how much is missing. From my own venerable A-1:
Is it just the little crank on the top? Or is it the knob that contains the crank and/or even the spindle below the knob?
Somewhere online years back I got an A-1 service manual. It's 200 pages are mostly more than I wanted to know about the electronics, but I am linking two pages that show some of the many parts in the area of that rewind.
It's not all that evident what is happening there (and I'm not about to take my A-1 apart for better look! ) but I think that spindle somehow picks up a link that disengages the door lock. If the spindle is out, maybe some very careful work with a small hook, like a crochet needle or a cleverly bent paperclip would allow picking up the link enough to release the door. The inside of the A-1, with scads of flexible circuit board wound around the interior, is something I would not look forward to digging into.