Firstly, Welcome to ribnet!
You pretty much hit the nail on the head there - 50% more power = more grunt to get on the plane. I can't give acual comment on that setup as I've never used one like it, but if it's OK-ish at the moment that should do the trick. (I went form 25 to 30 Hp on the back of an SR4. The time to plane was substantially reduced with a moderate power hike).
As for newish 2-strokes, I guess that is a popular size. Polwart has one, so they were made, but 20 is a bit of a no mans' land. Too big for generic cart-about-able SIBS, and too small for smaller Ribs. Hence I suspect not as many as those around and who have one want to keep it. If you can be patient, one will show up eventually.
Also any reason for 2004/6? To put it in perspective the museum on the back of mine is an early 80s machine according to the paint job. The desgn started life in the 60s..... Put another way a 2006 engine may be the same lump as one built in 1999 with a different paint job.
The thing with outboards is a 3YO one left out and driven with the wrong prop for all that time could be in far worse condition than a 10YO machine that has been looked after.....