Originally posted by grahame You will have to find a unit as same size and shape. Otherwise the focusing screen and viewfinder will not be aligned. And be careful about the exposure light sensor.
Since it is a Penta-Mirror, I assume that it is made up of five separate mirrors, unlike a Penta-Prism, which is a single block of glass, with five faces precision ground onto it.
I have to believe that the positioning, as well as the size, of the five individual mirrors is critical and that factory has special jigs to hold them in place, while it is being assembled. These jigs would not be available to the public, and would probably be difficult to fabricate, especially without knowing the exact angles and dimensions of the mirror assembly.
So, my not-so-educated guess would be that, no, a pentamirror is NOT replaceable, at least not by anyone lower in the parts chain that an authorized repair facility. Like many other repairs to these cameras, I strongly suspect that it would be cheaper to buy a good, used camera.