it might be water in the fuel. ethanol attracts water which sinks to the bottom.
just open the float drain plug and get rid of the fuel, then top up with fresh and put an inline fuel filter in too
your looking for a code something like 6220rs measure the outer race and the spindle for a size and cross reference that way.
don't forget there's usually a mushroom spacer that fits inside the sprocket carrier bearing (on the inside) and peeps throw that away with the bearing. and yes you need it as one bike I serviced for a friend had it missing and the bearing was about to fail
tip the head upside down and put petrol/turps in the "bowl" if it drains you need have the valves ground (try those kits first) and retest. if it still does it, the seats will need recutting by an engineer
cheers, It carried Tommy and myself down to Shropshire the other week. 80 mph on twisties and felt great.
Just got to keep an eye on the oil as they do like it,
went to Wray tonight , did about 150 miles round trip and It got a bit cowd later on. My core temp was really low by the time I got in. Took an hour to warm up
John McGuiness was there ( nothing official just chilling) and it was a really good night.
Went with Billbot (RD350) Brad (350) and a couple of other guys
a long way, great roads though
https://www.google.co.uk/maps/place/Bishop's+Castle,+Bishops+Castle/@52.4922605,-3.0064525,15z/data=!3m1!4b1!4m5!3m4!1s0x48700ffd3b29af49:0x2702b017b6bfc73a!8m2!3d52.4900507!4d-2.9984386