It's worth what you think it's worth.
If you need to make a grill and replace all the speakers, it won't be cheap. If you need to replace 2 or 3, ya might as well replace all four, because you have no way of knowing if new and old will play nice together.
I have no idea what drivers are right for this enclosure.
Best of luck.
What kind of beer do you like?
Cold and free!