the usual cause of failure in the CB23 is using overly thick oil in the engine and running it dead cold, this causes low oil flow to the cylinder head, When you take off the head you will notice an oil jet in the cylinder block, this oil jet is very small and is placed at the factory to have more oil "flow" to the crank compared to the head - It also ensures rapid oil pumping action to the crankshaft when the engine starts.
This alone dictates a lower viscosity requirement for engine oil to allow lots of oil to flow to the bearing parts of the engine, using a stiff 20W50 may cause problems and put extra load on the oil pump too. The operational temperature of the engine is critical too, it should warm up nicely and evenly to 82C. You will also notice there is no PCV valve in this engine, it has two PCV pipes one is on the oil cap which itself acts like a catch can and it works at idle speed, the other pipe goes into the air cleaner box and gets suction when you accelerate the engine.
I had come across quite a no. of CB23 engines in pk where the ustaad RA had hogged out that oil jet assuming higher oil flow and some also simply had hammered a rubber plug in place of the oil cap (no threads on CB23 oil cap) because they could not find a replacement oil cap - a new one was about Rs 400/- when I lived in Pk.
Not maintaining the PCV system or having leaks in the engine can cause the oil to start sludging (very quickly if the engine is run dead cold).
After such mishandling the chain reaction of cam wear, rocker shaft wear and noise starts.