I am solidly converted to 4 stroke outboards, a huge advocate, but still have a healthy respect for some 2 strokes. The 40 is one of those. So I'm thinking 2 things. First, maybe seek some help getting the correct prop for your 40. Look into cupping as far as a partial cure for the cavitation issue. Maybe even going with an inexpensive alum. 4 blade.
That will allow you more time for your 4 stroke search. With no experience, I'd be inclined to stay with motors that have been run in fresh water only. They are really easy to be burnt on, even if they appear to be running correctly.
You don't see them frequently, but there ARE 25" Honda 50's and 90's out there. I just did a 25" to 20" conversion on a pair of 50's this past summer....
Last, I have no idea what your boat looks like, but I can tell you that the Honda 90's are pretty heavy. The fact the boat currently has a 40 on it makes me a bit nervouse regarding the idea of putting a motor on the back that weighs twice what the 40 does, even if the boat is rated for 90hp. If the boat is an older one, the 4 strokes weren't around much back then, and there was no thought about all of the extra weight involved with a 4 stroke when assigning those max hp numbers. A Honda 50 might be a much better plan.....