The PCV is the Positive Crankcase Ventilation system. It uses the engine's crankcase as a vacuum pump. As the piston goes up and down, the crankcase effectively increases and decreases in volume. The PCV valve is simply a one-way valve that lets air out, into the air cleaner, but does not allow any back in. The only way air can get into the crankcase to replace the air pumped out, is in the form of blow-by from the combustion chamber, leaking past the piston rings. The whole purpose is to ensure that blow-by cannot escape to the atmosphere, since it is rich with pollutants. The PCV system is not related to lubricating the engine. Most mower engines do not have a pressurised oil system: they just have a rotating slinger or a dipper on the bottom of the connecting rod, to splash sump oil around the crankcase.

You need a diagram to help you assemble the clutch parts. Note which way up the Belleville spring goes in: if you put it in upside down, the clutch will be permanently disengaged and the blade will not rotate.

Here is an illustrated parts list showing how the whole mechanism goes together, including the return spring, washers etc:
[Linked Image]

I haven't actually seen this mechanism, but I'm wondering if you can assemble it by first unscrewing Item 5 on the diagram, the bolt which holds down the bracket, Item 4. You may be able then to assemble all the parts with the spring uncompressed, then compress it by doing up the bolt (Item 5).