Questions Converting a Long shaft to a short shaft lower unit

[Kongfish]

I have a 2011 Mercury 90hp Four Stroke EFI outboard with the X-Long 25 inch shaft lower unit that I need to Convert to a 20 inch short shaft lower unit. I have two questions about converting the lower unit over to being a 20 inch short shaft.
A.) would I be much better off replacing the entire lower unit with a intact whole lower unit with a 20 inch shaft lower unit? or
B.) Would buying all the parts needed to do the conversion job be just as reliable way to get the 20 inch shaft lower unit ?
Can anyone provide me with a link or a download of the correct shop manual needed to swap out a 25 inch shaft for a 20 inch shaft? I need to see if this is a job I feel like I can finish correctly?

Can you provide me with a complete parts list I will need to do the conversion correctly?
SERIAL #. 1B853167 of my motor.

 

[Chinewalker]

Looking at the parts list online, it appears your motor has a 5-inch extension housing, along with longer shift rod, driveshaft, etc. Might be easier to find a 20-inch shaft lower unit and remove the extension bits & long shaft unit from yours. Otherwise, you're likely paying someone to swap shift and driveshafts.