default wrote:

Has anyone ever replaced a window? Mine doesn't look like mica but
does look like some sort of hard mineral. It is only fastened over
the opening with a couple of push in plastic pins and looks like it is
intended to be easily replaced from the oven chamber without taking
the whole thing apart.

The window - or waveguide cover as it`s called - is available by the
sheet. It`s either laminated mica or fibreglass[1]. You simply cut it to
shape using the old one as a template. pierce the holes and replace the
plastic rivets. Clean any grease and carbonised crap out of the
waveguide while you're in there.

[1] Some ovens have a purpose made moulded plastic cover which may or
may not be available.

Ron(UK)