Try shifting the gears by hand while turning the drive sprocket (or shaft) to help them move. The C2 gear might be sticking out too far? If this doesn't let it slide inward far enough, then the forks inside might not be on their respective gears? You can check this with a flashlight, thru the oil pan. A little hard to see. In particular, if the "C" (Center) shift fork is not sitting on the groove of the gear on the mainshaft, it can cause the others to not line up very well. This will also cause loss of gears 3,4, and 5.
