You can get used but will find you'll probably need to buy new/used parts in addition (gear box, shaft, strut, prop) to make it work for your application.
The better used deals I have found usually include the boat, motor, and trailer. That's why people make money parting boats out. It means you'll end up with more stuff than you want but will save on the front side.
The notes section of the Monsoon do not call out specific power/gear/prop needs so hopefully that's covered in the plans. It does state, however, speed performance based on SHP. Going by 24hr performance I'd think desiel is the way to go.
Kens is a pretty knowledgable with all this stuff for this size boat so hopefully he'll chime back in here.
But, in one of the photos (below) I see what looks like the top of a I/O leg...this could be a possibility as well and probably easier to find in a salvage operation.

Looks like a neat possibility...just thinking out loud here.

_________________
Bill
I told my wife we needed a three-car garage for my projects...she told me to ask her for permission next time before I buy a house.
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