Hmm, wonder if that would be covered by your car insurance "glass cover", can't see it being easy to find a replacement !
Looks like there isn't really enough overlap between the frame and the glass for a good bond, I guess this predates a lot of bonded windscreens etc, still a good modern windscreen adhesive should do the job.
I think part of the problem will be the amount of flexing it gets in use, the roof frame (both the car part and the roof part) aren't particularly strong, with the roof removed you can twist them by hand. Nothing like the massive structures you see on modern cars. Add to that any flexing imposed by bolting it to the frame.