Best I can tell, I possibly need either a new shift fork (#4)
or a new #16,
which rotates on the shaft to slide the fork "fore-and aft"
View attachment 521612
Have you tried to adjust the turn buckle above the "L" that attaches to #24,
or even taking the lever loose there and try rotating #24 by hand to see if it engages?
To physically see #4 or #16 it looks to me that the top cover and 3 pt housing will need to come off.