Removing a window from 1930s era building
I haven't worked on vintage metal casement windows but have some experience restoring vintage wood double hung windows.
First, don't replace the entire window unit, I'm sure this window can be repaired for much less. Did you contact a company that sells new replacement windows? I would avoid those companies and try to find a company that can re-glaze the existing window. Maybe search for a local glazier that specializes in window glass replacement.
Are you able to open the window that's broken? If so, maybe there is a way to remove the brackets from the frame instead of removing the brackets from the sash. Sounds like access from the exterior would be challenging, so if you have to do this from the inside then you'll need to be careful to not drop the sash if you can get it loose.
It may be possible to replace the broken glass and re-glaze the sash with new glass without removing the sash from the frame. Since the glass is broken you could try removing all of the broken glass and then use a heat gun to remove the old glazing putty from the metal sash. Once that's done you just need to install new glass and re-glaze with glazing putty designed for use with metal window sashes. I've used Sarco Dual Glaze putty and would recommend it for this. Do not use DAP 33 glazing putty that you will find in home improvement stores since it takes forever to dry and is messy to use.