How sad! Seems more like vandalism or perhaps an accidental impact than a natural failure.
The window itself is just plexiglass, available at any hardware or home supply store. Take the broken piece to match the thickness, and use the old piece as a template to cut the new.
Given the age I’d probably replace the gasket, too. That’s available online, and you have to know the thickness of the window and the surrounding fiberglass. You”ll cut it a bit long so you don’t get a gap at the bottom center seam.
The process of getting the old one out and the new one in is doable but a bit tricky. There are threads with how-tos and tips. I do recall there’s a tool you need. Haven’t had to do mine, so I’ll leave the details to others, or you can search for those old threads (site search/google at bottom of search menu gives best results). Try “replace Scamp (Boler) front window.”
If you decide to hire it done, some auto glass shops can do this type of window installation. It’s common in heavy equipment and vintage cars. Ask around and make sure they have actual experience.
You can purchase new upholstery covers from Scamp, but the fabric has changed. As an alternative, maybe you could deconstruct the top bunk cover, use the underside to patch the bad section, and use a complementary fabric for the bottom, which doesn’t show as much.