The X-ray window will likely be a thin aluminium or beryllium sheet. Beryllium can be nasty but its fine provided it is intact. This window will also be a vacuum seal for the x-ray tube. It's very fragile and if you break that it is game over for the tube. There is quite a chance that the vacuum in the tube has degenerated with age and that the tube might arc over and not run at it's stated voltage.
Last thing to point out is that the fluorescent signal will be very weak and emitted isotropically so even with a very fast lens and decent modern sensor if you move further away the signal will go down. This is the main reason the fluorescent/scintillating screen is bonded to the sensor.
Hope it's some help.