I can believe it can be the source of "A" leak. You'll have to check and see if it is "THE" leak. You can use foot powder or K2R spray. to find the source. The oil will get soaked up and leave a trail in the opposite direction of gravity. Usually at the top is the source. If it is the cam cover seam, then the crack aren't allowing the proper squeeze on the cover gasket. But, the rocker inspection caps can leak, too. How old are those Orings? You do know that those fined caps aren't stock, yes?
The eBay pointer is for a 76. It is technically the correct part for your bike. The picture is not of sufficient quality to determine wear patterns on the rocker cam follower faces. When the cover rocker shafts turn in the soft aluminum cover, the supports can wear unevenly, allowing the rockers to Kant on the cam lobes, resulting in uneven wear on both the cam lobes AND the cam followers. They don't all do this. But, some do. And, you don't want one. (But, if you do I'll happily send you one!
) Anyway, this is why I recommend getting a 77-78 cover the redesign prevents both cover wear AND cam/cam follower wear. You may well find your existing cam and followers damaged once you open it up a take a look at them.
If not too old (more than a year or two in use) the cover gaskets can be reused. But, like all rubber, it hardens with age and becomes stubborn in the sealing department. If you can easily dig your thumbnail deeply into it, then it can be cleaned and reused. But, the cross section should be round, if oval, replace.
Yes, all top end work can be performed on the CB500/550 with the engine in frame. There is a service warning about installing the cam cover you would do well to heed. Wouldn't want to bend all the valve stems, now. $$$$$$
Cheers,