My Z3 is around 25 years old. Around 10 years ago I got a back screen from a seller in germany on that well know auction site. It was about 80 quid if I remember rightly. Came with the zip clasp thing and old one came out and new one in in less than an hour. Has been perfect for the last 10 years.
But (there's often a 'but') just a few weeks ago I found some of the original 25 year old stitching was starting to break. Tried re-stitching but the more I fiddled the worse it got.
So, I would say, if your car is as old as mine then you might find some of the stitching is rotten so won't take the new window well. My car has been outside for many of those 25 years. Daily driver all year round so maybe sun and weather hasn't been kind to it.
But just wanted to have said that a whole new roof, fitted, isn't that expensive.