You may have many more concerns than Hehr windows. I suggest that you do some research and check out some...many...photos of where you want to go and where your at. The back window will be the least of your work. It was plexiglass and made from flat material. Lexan is your best bet on replacement....then there is the wood bracing inside and the type of rubber seal you can find.
"If it can't be reduced, reused, repaired, rebuilt, refurbished, refinished, resold, recycled or composted
then it should be restricted, redesigned or removed from production."