Estimated, in simple words I believe that your problem does not lie in what you specify (or cylinder head, custain, camshaft) I remind you that the oil circuit begins from the carter (where the oil is deposited) the tube that sucks the oil and that it connects with the oil pump, the oil filter, the bulb or oil pressure sensor and then to the rest of the engine. Personally, I would begin by checking the proper functioning of the bulb or oil pressure sensor in the face of the doubt (for this type of engines they are not very expensive) if not, check the valve that is where you screw Oil ... Good luck