I haven't got a diagram available, but it is quite an easy job. Its best to remove the transmission unit after removing the top cover of the machine ( for easy replacement of the transmission) If you read previous answers I have posted on Kirby, you will see in detail how to remove both the top cover and the transmission.
The neutral drive pedal with the cam lever unit attached should be installed with the lever in the drive position. The finger will fit in between two sections on the main axle of the transmission, and the u shaped metal on the other side of that cam lever pivot is moved when the pedal is moved to the neutral position.
The cogs on that axle are separated by this mechanism as neutral is engaged. It is all spring loaded and as drive is engaged all slots in to place. The small screw holding that cam lever pivot unit must of course now be replaced to hold all this together before replacing the transmission unit. Be certain to engage the fork on the top left of the transmission over the stud which ultimately is connected to the main handle. This operates the forward and reverse clutches. That is why I recommended removing the top cover of the machine. You can see everything clearly with that removed. Thats it.... it will be easy !
gj, June 2011