I originally had a bad tranny when I brought this bike. Long story. Anyway I rebuilt it and installed new gears on the tranny and a new shift drum. I put everything back correctly and before starting it went to put it in neutral and no movement. When I installed the tranny in the case it was fine and rotated against the other shaft. I put the forks on right too and in the groove of the shift drum so why won't it shift? I would hate to have to pull it all apart for something simple. I already took it back out of the frame and took of the cover over the shift drum. That was when I noticed it wasn't moving. I tried tapping it with a brass dowel, but it only moved .060 about. I can't rotate it anymore. Even when I was using the clutch I had no movement. I just don't know if it is something simple. Hate to redo it and spend all the time on it and money for gaskets. It costs over $200.00 just in gaskests.

Have had some experience with this, if your 100% u put it back right, then take off your clutch cover and try to move selector drum manually with lever being careful not to lever on any thing weak or important .
If not pull it down have a good look, be gentle with your gaskets if they are fresh you can save them, try a razor blade.
Make sure they are the right parts.