Welcome to RC Lindburg conversions.
Larger servos make VERY good motor controllers. They have the built in F & R.
I used to keep the speed pot for setting up NEUTRAL anywhere I wanted to. A LIGHT spray coating of clear coating did not affect the PCB board of my HITECS . The rudder servos can use a grease blob on the output shaft of your rudder servos. Put enough on, so that when you push the arm back on, there is a water proof seal..
I ran every thing off of the receiver battery. Very simple & O K if there are NO WEEDS around.
They ran fantastically. My favorite was / still is the L C.
Drive shaft were small diameter pushrod wires used for servo connections. I found that the servo motor from STANDARD SIZED servos could sometimes be used as the drive motors. i depended on how long the motor shafts were, At times I would use the gears with a inner & outer teeth set . I would press the large gear side on FIRST. Then use the small teeth to slide rubber tubing onto for a coupling.
Lots of thinking & modifiying for you to do.
L C boat had cargo boxes added to it. I made them to cover all the batteries rec. servo PCB.
All is fair in small models. Enjoy the challanges.