I am confused. No worries, it's easy to do. You had 3K to 4K for a boat, you bought a boat for ? $600 ?, it needs a $500+ place divertor/pump parts, it needs a $700+ interior, and now there's 5K for an engine?
Secondly, there's no replacement for displacement. It takes a lot of torque to move a jet boat, any big block will move the same jet boat with much less stress than any small block. A mild big block will have the same HP as a pumped up small block and the big block will be more reliable and use less gas than the extreme small block. My buddies Chevelle in high school got 6 mpg with his first 383 build and 4 mpg with his second 383 build. He wrecked that Chevelle and bought a SS396 Chevelle, that one got 10 mpg and was faster than both of those 383's.
Does your engine builder have jet boat experience? Think about it.