With the lenses removed, it's clearly visible that a new surface mirror is needed. To get the old mirror out, the waist-level finder needs to be removed. This is necessary anyway as the ground glass is dirty and needs cleaning. And one piece of felt fell out of it when I opened it for the first time (there are four pieces around the ground glass).

The finder is attached to the body with 6 screws. Two on the top behind the finder and two on each side. On both sides, one screw is at the front end of the camera and one is hidden behind the camera strap holder which thus has to be removed at least halfway.