I never heard of a navy top. Old tops snapped to the windshield were called convertible tops. If it was free-standing it was a bimini. Side curtains were made out of eisenglass, and they were called side curtains. The back was called a stern curtain.
Plexiglass is a solid plastic that was used for windshields, or you could buy it in the hardware store to fix your screen door in your house.
Lots of different terminology. Google the words and find out, but don't rely on internet suggestions.
To answer the OP's original question, you got an old boat and parts are not available. Find a marine canvas dealer in your area and he can make you whatever you want.