Roof panel mate.
As Oranoco says, get a good bodyshop to fit it or it will look crappo.

Someone off here actually replaced the full roof skin & posted pics a while back (prob last yr sometime) but can't remember who it was. Looked quite a scary job, but was very good when finished.

Finding a new shell with good roof is a bit extreme, unless the rest of your bodywork is getting shabby as well.