It is quite possible to have the plastic rear window replaced with new plastic. Boat canvas places sometimes do it. Ask at Porsche dealers where they send their cars.
It is also possible to remove and replace the top yourself including with one with a glass window.
Some reading just to see what is involved is
here . Be sure and read all three web pages on the subject.
Being a new owner, you also might want to poke around on the site to see the other resources there. It's worth a bookmark as it has at least 30 of the most common questions answered or issues discussed with writeups based on the collective wisdom culled from reading far to many Boxster forum postings.