I found this article that talks about the option of just buying the jet gear for my current motor:

https://www.boatingworld.com/engines/drop-the-prop/

They claim you can switch them out in an hour, though that probably means at least a half day for someone of my mechanical ability.

I gotta research it a lot more, but this might actually work for me. Fishing rivers is a mid-summer through Fall thing, and the rest of the year I'd be better off with a prop. Switching it out once a year would not be that big of a deal If I could learn how to do it. My current boat would not be the ideal jetboat, but I think it would work. It has the typical modified V bow, but it is just a flat bottom at the stern. It's welded and the only modification I think I would have to make is a jack plate. It runs about 32 now; if it only ran 20 with the jet on that would be fine. I have found that access to 6-8 miles of river is plenty for a day of fishing.

It looks like I could get everything I'd need for under $3k, and that's a lot less than the $18k or so a new jetboat would cost. And I'd have the versatility to make it back into a prop boat for winter and spring. But it may just be another bad idea.