The vinyl roof was a Fiat production item, it's not an off the shelf option. They didn't use webasto like Minis did, they used specific roof section to strengthen where it was cut away. It would be a difficult job to get right and it won't be cheap.
If you can accept a simple tilt glass sunroof they are much easier to fit, I had one on my Peugeot 106. A hole is cut in the roof and the frame bolts through from either side, it's designed to add the strength back. Mine was old and started to leak so I stripped it and re-installed the frame using windscreen adhesive to replace the failed seal. It never leaked again and it still opened as it should! you will find one in a scrap yard.
That said, I would be reluctant to put a hole in the roof if I didn't have to!