Twin 150 Mercury 4 strokes would be awesome on that boat. They are the most popular engine in South Florida right now. I know guys who have sets of them on 28 Whitewater's, Conch 27's, a 31' Yellowfin, 24 Browning, etc. etc. The guy with the browning actually had his boat sink. He had to replace some bolt on components, plugs, etc. but a year later they are still running perfectly.
I know the Whitewater and Yellowfin (now called the 29) both top out around 46 mph. The Whitewater gets upwards of 2.5 mpg, loaded down probably closer to 2.0. I have seen them consistently for around $10,500.
If you were thinking new this is one of the most economical and reliable outboards around. 5 years warranty I believe.