1. Use a Graupner 700BB 8.4v
2. Use 12 cells.
3. Buy an appropriate ESC to handle ONE 700 motor. The Jeti Navy 700 works well
4. Set up One surface drive setup with a 42mm prop
5. Buy an appropriate hobby grade receiver.
6. Set up a rudder with a standard servo. Nothing is worse than vector thrust.
7. Get rid of the stock motors and especially the receiver/esc!

That's the best and easiest way to get 25mph without going brushless. What ever you do, get rid of the vector thrust.

Ryan