Black plastic piece

Thanks for posting. This is called a Top Sash Guide. There are many different kinds, and it's admittedly a bit tough to identify yours without seeing a clear photo of an undamaged version. The first that comes to mind is the Harcar 18-172, but compare the diagrams below with a whole version of the guide to see if it matches what you have. If it doesn't, please send 'mugshot' style photos, as below, of the undamaged guide, and we'd be happy to try and find something better.

Detail of 18-172

Side view of 18-172