Like others have said, it needs to be removed as an assembly but first you need to remove capscrews holding retainer #19 in your picture.
The bull gear stays in the main case,
I found it easier to reinstall by filing a chamfer on the end of the splines where the shaft reenters the bull gear.
You may be able to remove the retainer bolts by removing the PTO assembly but reassembling will be a challenge because the bull gear will drop a bit once the shaft is removed. I removed the top cover when I did mine. I kind of wanted to see what else was going on in there anyway.
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