175 Zukes is as much as I'd want to try putting on this one. That or two of the new 3.0L 150 Mercury 4 strokes. Supposedly they are pretty strong and reliable motors.
The Mercs can be had for as little as 20.5k. Pretty cheap for two 150 counter rotating 25 inch four strokes. Add in controls, the merc monitors and install and you're still around 24 grand.