Just for fun, I had two Pen Fs apart today. I have one good one but it's viewfinder is dirty, one with a dirty finder and the film take-up spool is really stiff (film will tear rather than rewind as it is so hard to turn), and one jammed solid parts camera.
So I too the parts camera apart first as there was nothing to lose. Got the top and bottom off, removed and cleaned a mirror under the top housing, and cleaned the surfaces of two prisms I could get at (to get at their other sides would have meant a much bigger job). Got it back together, but had to re-solder the flash sync connection as I was careless taking the top off. Result: viewfinder markedly improved, but of course the camera is still broken.
Moved onto the stiff one. Used a drop of lighter fluid at each end of the film take-up spool and it became much easier to turn. Could have done that without undoing anything, but I decided to do the same thing on the viewfinder as before. Didn't really make much improvement there, perhaps the dirt is on the surfaces I can't get at. Kept the flash wire attached this time, and managed to re-assemble and all is working.
I don't think I'm brave enough to do the one good working model (with the dirty finder), or maybe I'll practice again on the parts camera and get a bit deeper into the prisms first.