Welcome from sector 2,
I'd drain the trans oil, check the magnet for shavings off the fork/dogs? Or, showing a metallic oil drain with a lot of sparkles, we'll call this obviously a rebuild by oil contamination alone.
If I wanted to test for a pop-out-of-gear scenario:
a. Back wheel is off the ground and everything is static from here.
b. Hand on the shifter, other hand on the back tire.
c. I'm going to shift the trans as fast as I can so I am acting the toe and letting go. And that's from 1st to top gear, then down to 1st again. That says the pawl adjust is close enough. I have a problem zipping thru or to 3rd... There you go - Said Scenario.
d. Meanwhile, the back wheel is moved up and down, rather than hand spin a 360°.... won't shift as fast.
e. From 1st, I aim for 3rd and stop. (1)-Front wheel is up against the wall [2x4] and here is where I pull the clutch in so as to get some spin on the tire and pop the clutch. I get chatter of say a gear slipping into a few open holes = It's fucked. (2)-That was my first test. Now I need the wall where I'm going to push that wheel with all I've got and either the engine moves or the trans is the weak link and again, chatters out of gear = Tear Down.
Edit:
f. (3)-I could jam a round shaft type screwdriver under the tooth and link of the lower primary chain so that locks the engine from moving, then with two gorilla arms, snap the back wheel up hard so it loads everything and if that's what it takes to chatter the matter, I say no to loading the link to tooth in said manor. Avoid collecting more parts to the list. You know it's the trans, not the clutch, your shifting, the pawl, etc.
