I'd have to say that I've probably run out of usefulness on this.
I'm not sure how the oil feed is arranged on this engine, but, if you could lift that cam bearing and see it, there might be some chance of cleaning it out without completely lifting the cam. It is, however, more likely that you do have to lift the cam in order to get at it, because it is hidden below the split line.
(If you can still get them, a pipe cleaner might be something that you can feed along the oil way, assuming that there is a soft blockage somewhere near the point to which you have access. That may or may not help. What I was expecting was a cylinder head liberally coated in a black, thick, greasy substance. Given that there isn't much thick, black, greasy to be seen, maybe there is a completely different explanation.)
In which case, you do have to lift the cam, at least a little. Maybe there is a possibility of lifting all of the cam bearing upper halves and having enough play to get at the oil feed or the followers without completely losing cam synchronisation (which would make the putting back together easier), but I don't know, I've never taken one of these to pieces. On the other hand, the level of care needed to keep the cam in its position with the drive teeth may just be more trouble than it is worth. But, ensuring that you have the cam timed correctly is a bit of a pain (a fiddle, rather than excessively difficult, and the kind of thing that you worry about having done correctly), so if there is a way around that it might be a short cut. Maybe you can make little alignment marks that you can work to during re-assembly (re-assembly is the reverse of disassembly, as the Haynes manuals always, so irritatingly, say).