Replacement dated glass is available. I understand there has been problems with the curvature of repro rear windows.
To answer your question of OE vs. replacement. If you plan on driving and enjoying your car I would go with new glass. If being a purist and having your car judged as original and you are just plain bothered by the idea of replacing original parts, then dont replace.
It is very subjective and no clear answer. If it is an April car, I think I have a whole new tinted set?
I just want to add, I go to cruise ins toward the city. I am driving West right into the sun. An old glass really has a lot of glare and looks sand blasted. I don't care for that.