The earliest C series equipment is nearly antique (> 50 years old). What tended to improve over time was contrast due to changes in coatings. Most of the lens designs did not change (exceptions; 105mm D/DS, 180mm Super, 80mm S). They do prefer good lens hoods.
Causes of poor performance:
Focusing error, either due to poorly adjusted eyesight or a mis-aligned screen. There's a small chance that the lens panel has been damaged, but we'll stick with the simple stuff.
Lens pair mis-adjustment. The viewing lens is usually shimmed with small spacers to match the taking lens. Taking them apart can cause the shimming to be lost.
Fungus or clouding in one or the other of the lenses. Usually causes a loss of contrast rather than true unsharpness.
You need to do a fence test, I think. Find a fence with vertical railings, preferably close set. Put a mark on one and make an oblique pucture with the marked post in the middle, focusing carefully, and at maximum aperture. Ideally repeat the process from the otherside. Make sure you are using the flip-up magnifier in the waist-level finder. If you cannot focus the edge of the central focus aid sharply, you cannot focus the camera. If you have trouble with the flip-up magnifier, try a loupe direct to the ground glass.
Aims: ensuring you can focus the camera correctly, and testing to see if the taking lens focuses in agreement with the taking lens.
The first rule is not to fiddle with any adjustments until you are sure that you know which of the elements needs adjusting. The second rule is only adjust one thing at a time.