Re: 25 inch shaft to 20 inch?
If the 25" motor has the 5" spacer and your 20" gearcase is good, you should be able to swap them. You will need to swap the studs on the midsection as well. The watertubes should have 5" extensions you may remove. I forget what you need to do about the shift rod, but it worked for me, when I did my swap.
You may be able to swap the powerhead instead.