Just to be a spoil sport [sorry], do you think it might be partially seizing--tightening up in the bore. Sure hope not, but if you drop the exhaust & look up there you'll get an idea if all is well. Could take the reed block out to see the other side while ya at it. Certainly hope not, but good to eliminate that.

Here we have an honest man.

Yep did read that, which made me think ok--but was it completely out of oil [for the original seizing] or was [or is] there an undetected oil pump issue. Anyway, hope OP gets to the bottom of the problem whether lean/lacking fuel flow etc. Or did OP go over a decent bump & a lump of errant carbon blocked the tail pipe--or spark arrester in the tail muffler, just saying.

Actually i'd qualify for 'dumb' thinking back to my "72 XA Ford Falcon. Got very rattly on a steep climb coming home from an MX meet in the day. Stopped at the hilltop & couldn't do anything to stop it although it got a bit better on the flar running & only half of them rattling. So, next week i got the lifters out & head off [probably didn't need to take head off] dismantled 12 lifters & nothing obvious [could jack the car up with them leaking down slowly]. End result was motor was almost out of oil & that's all the problem was. [do i win lol]. Lots more similar stories too.

Did you confirm the oil pump is working, sometimes when the pump goes dry you have to bleed the air to get it to prime and pump again, just a guess, think most of the rest of my guesses have been covered by other members.

Carefull, that maybe construed as a sexist remark

Thank you all for your messages and awesome help! The oil pump was tested and in perfect working condition, in fact before sending it off to my mechanic to re-bore the cylinder I did a total tear down of the bike just to make sure it wasnt something mechanical that failed to work. The reality was that it had been "user error" and I was the only one to blame. My thought was that it is either rust that slipped through the filter and clogged up the carb or air that somehow got into the carb. I opened the main jet and let it bleed a little bit like some of you suggested to see if that works. I also just dumped a bunch of evaporust into the gas tank to try and alleviate the rust issue. Fingers crossed this helps!