Clutch seized while running

sounds like your clutch hub bearing is bad - is there a round mark inside the clutch cover, or are there fine, steel wires sticking out of your clutch pad? - either of these can put metal into that bearing

Yup... if you got the clutch pads that look like they were cut out from an old tire, they do she'd off tiny pieces of fine steel wire... not good for the bearings. You can trim off any pieces of this fine wire before you run it as preventative maintenance but swapping over to the brick red hard rubber pads will prevent that problem for good, and this type does grip really well, or at least we'll enough to hold on a stock engine or one that's mildly modified, I've never had a clutch slip issue with the red pads and some of the engines I've built have a lot more torque than stock.
it is possible that the clutch hub bearing seized up and just needs replacement, or it could be a bad clutch cam on the other side, or the bucking bar or steel ball. I would remove the clutch gear cover and check there first and check for metal shavings or pieces of fine steel wire coming off the pads or in the clutch housing, then check under the actuator cover for a bad or worn clutch cam or bucking bar if all looks well on the hub and gear side. Clutch parts are fairly easy to replace and not too expensive.

I've also found the clutch shafts bearings to be loose and rough turning on some engines just after about an hour or so of riding, these bearings can put extra stress on the hub bearing if one of the shafts bearings gets too much play in it, making it near impossible to adjust enough for a full disengagement because if the shafts bearings have play in them the whole shafts will move a little before the clutch will disengage. These bearings are the same size as the crank bearings and fairly easy to replace, a set of sealed SKF bearings will last longer and eliminate any shaft wobble or side play so I would recommend changing these for quality bearings if there's any signs of wear on either shafts bearing.

the inner disk opposite the flywheel freewheels and will turn a a different rate of rpm than the pressure plate/flywheel likely it should be welded so both pieces spin at the same time

I'm guessing from the pic of your bucking bar that your ball bearing is in there - I'd be taking the chain of and turning the hub to see how that bearing feels

was there a small circle worn into the clutch cover?

as I mentioned, it looked like you had one ball already in there (feeling in there with tiny screw driver will let you know if something round is inside) - to get it out, you may have to loosen flower nut and tap it with a hammer to see if you can knock the ball out to other side - this is a bit easier than other way of taking shaft out and banging it on something hard to knock it out

easy to get fooled if you don't feel around in there - only the ball bearing will feel smooth & round, if it's missing it will feel rough & flat

shaft can be pushed out toward clutch side once drive sprocket is off

first the nut & washer come off, then clean the threads inside with a rag, then very carefully thread the puller tool into those threads

then put the nut on loosely to protect the threads and use a mallet to drive the shaft thru to other side (removing nut once it is moving thru)

just had a guy this AM - stripped off the woodruff key under drive sprocket and tore up that whole shaft so there wasn't even a slot left for new key

tapped the shaft out with clutch intact on other side (because I couldn't grip the shaft to undo it) - slid right out with a few taps with a big mallet

then I was able to grip messed up end of shaft in vise to undo clutch and move all to good used shaft

just finished an hour ago

Yup... there's a small woodruff key that goes in there... These things are really small and easy to lose so I try to keep several spares, but you may get lucky and find it on your garage floor, it's the typical half moon key and it's only about 3/8" long. You can get replacements off ebay or thru the venodrs on this site, but that means having to wait a few days for them to arrive. If you have to order a new one, get about 4 or 5 of them and store them in a small pill bottle or something to help prevent loss.

drag a heavy magnet around the area where you were working & it may appear

in a pinch, you can make one by folding a steel washer in half (maybe 2 if thin), but better to find a real one - some hardware or auto parts places may have them