Yes, the rod bearing was a really common problem with the nighthawk 650. Hell, it was common with a lot of 80's and 90's bikes (ZX-11 included) I had a couple of them that I sold for parts not that long ago and I spun the crank on one when I was a kid.
You could try to pull the pan as I've heard of people changing the rod bearing from below (usually #2, I believe). But it's likely that the problem will just come back because the theory is that the oil journals just aren't big enough. Plus, there's a good chance that the crank could be damaged and require replacement or resurfacing. When the crank is resurfaced, that's when the different size bearings come into play. If they mill down the crank .001 or .002 to get it right, you'll have to compensate by getting a slightly larger bearing. Makes sense right? So, most likely, you'll have to pull the crank which will have you pulling the valve cover to get the cam chain off. You'll take the crank to the machine shop (or you'll have to use a micrometer to see if the crank surface got hosed), get the crank resurfaced, get the right bearing (one clam top, one clam bottom). Then you get to put it all back together and hope you get it right. Then it could be 60,000 miles before it happens again or it could be 6,000
Now don't get me wrong, I've heard of quite a few guys getting 80K out of a cb650 with absolutely no problems at all. Some people believe the problem only really occurs at sustained highway speeds. Regardless, if it were me, I 'd look for an 84 or 85 700s nighthawk in great shape or see if it's possible to put the 700s motor in your 650 frame (not sure). The 700s never had the problems of the 650 except when people rode wheelies for too long on them, starving the motor of oil.
