Bryan has one of the sources right: another can be leaks from the cam bearing studs, especially if someone took them out and didn't seal them when reinstalling.
Also:
if someone before you replaced the head gasket, they very likely introduced the oil leak by not installing thicker O-rings at the 2 oil ports at the back of the cam chain tunnel, on top of the cylinders. VERY common. You can also mill off the cylinder deck instead: I recommend both, and do that with all these engines I rebuild. This leak can be determined by WHERE the oil is coming from: if it is coming out of the front of the cylinders just below the exhaust ports, this is the culprit.
Also possible (but rare, have only seen it 3 or 4 times) the cam cover gasket may leak. This can be caused by a dozen things, including a warped, damaged, or cracked cam cover gasket surface, all usually at the hand of a poor mechanic.
