It sounds very much like a sticking valve or shim bucket to me. If it was a big end rod bearing it would sound more like a knock. A crank journal bearing doesn't knock and the oil pressure would drop like a rock.
I kinda thought that when I looked at the previous pictures of the top end including the cam chain, that everything looked pretty dry in there. Engine oil should have been pooled in the cylinder head cavities and the cam chain should have been soaked but in the pics I saw (which were really dark) things looked dry. I saw the oil pressure light go out but I question the "flow" of oil to the top end. The oil pressure sender on these engines is just off of the main oil feed gallery down near the oil pump and I find that to be a poor location To check for pressure. A better place would have been on the oil gallery in the head to ensure the entire system is getting oil.
At this point, I would pour a cup of oil all over the camshafts and shim buckets and let it sit overnight and then try starting again and see it the noise shuts up. If it doesn't, do a compression test and see if you have a low cylinder which would indicate a possible bent valve.
I too have concerns about the rust seen on the camshaft chain and wonder what else is rusted. This "test" mentioned above would be to assist in determining what needs to be done at this point. You could just stop all testing and tear the engine apart and inspect everything and repair what you have to. Testing just focus' the effort to repairing what you have to. Cam chain replacement would be prudent and it can be done without breaking the engine. I've done it. Requires buying a new chain and a master link, then splitting the old and new chain and then riveting the new master link once the new chain is wrapped around the secondary shaft in the bottom end. Also requires the purchase of a chain riveter available on EBay for around 20 bucks. Just a little tricky riveting the link without over riveting which would create a tight link.
Hope these thoughts help.
KB
