I would run a flex cable over wire drive.

90mm Rudder would work.

ESC - I would recommend 90A, the higher the better.

Prop will be determined after motor selection. Select the motor based on these [link=http://radiocontrolinfo.com/ElectricBoat/motor.php]Motor[/link] tips.

Scaling Drawings is not an ideal case. I can't remember if the plans have a dimension for that sponson distance. If not you may have to scale the drawing, meaning take the dimension as measured.

Ryan