Other than replacing the whole top, can the plastic rear window be resewn and replaced or is it just cheaper to purchase a new convertible top?

 

Provided the rest of the top is in good shape the rear window can be replaced.

There is one way to do this and there may be a second. The first way involves cutting out the old window leaving its border/edge in the canvas. A new window is fitted and sewn in. This creates a dual track of stitches around the rear window, the original stitch track and the new stitch track. This also leaves a bit of ridge in the canvas.

But the window is water tight and if you pick the right shop the plastic window will be the heavy duty plastic and last as long as the original.

The 2nd way is I believe the entire old window can be removed and a new window sewn in.

You'll have to find a shop and ask if the 2nd technique is even doable and what it costs. Three or 4 (or more) years ago I paid $700 to have the rear window in my 02 Boxster replaced. The shop used the 1st technique. I was unaware of the 2nd technique until I talked to a person at another top shop some time afterwards and the person to whom I talked to indicated the 2nd way is doable but I never confirmed this.