Are you trying to keep the back deck original? Or are you willing to kind of do your own thing?
I ask because if you want to keep it original, you're probably limited to stock replacements. Some sound pretty decent, although not great. But if you go beyond that, you're going to change the appearance of the interior. All depends on what you want.
There are many kits available, so if you have the size of the speakers in mind that you would like you should be able to get something going. You should always start there.......with the size........and build around that.