Unless you know the entire history of this machine it is probably the thrust washer next to 1st gear.
Lots of guys replace rear axle bearings, swingarm bearings or the rubber boot and they pull the countershaft with those 2 bolts on the left side, BIG MISTAKE!!
When they do this the 1st gear thrust washer falls out and into the crankcase and settles on the bottom. It can be retrieved and fixed without removing the motor, I've done a few.
Once that washer falls out, 1st gear continues to work but at partial engagement. Over time it gets sloppier and starts to grind or jump out of gear.
It is very rarely a shift fork, shift shaft, shift drum but there are some pieces under the left (stator) cover that seem to come loose or snap off. Your issue sounds like 1st gear thrust washer is missing... Just my best guess!
All our government does is distract us while they steal from us, misspend our tax $ and ruin our country