20 HP problem

[Jimmy M.]

Hi Folks............
I have a 20 MSHB....... 2003.....that's what it says on the tag

I need a lower gearcase and cannot find that motor on boats.net or any parts manual.....only 15 MSHB or 25 MSHB

So my question....would it be the 15 hp gearcase 63V-45311-01-4D,
or 25 hp gearcase 6L2-45311-02-4D ??

Thanks for any help with this.......Jim