Dear Mr. KWAKNZ,
We are thrilled to have you aboard! Let me try to shed some light on your questions as a newbie can.
I would like to make it full proportional rudder and motor.
To do this I need to change the rudder servo and change the motor servo correct?
Yes, change the rudder servo like this
To control motor, a) an on/off/reverse switch controlled by a servo is fine and you can pull out microswitches for this from any old computer or vcr
b)alternatively you can use pretty much any esc rated for 5Amps or moreAlso I would like to keep the original motor, but want longer running times, can I use the 6V 6AH AGM SLA BATTERY with this motor? and change everything else to work with this battery?
Absolutely, you can use the 6v battery, but the boat will be a little slower than stock speed. See I'm confused, but did get to take the boat out this morning for its first run.
Prior to your maiden voyage, check for leaks in the bathtub, don't without a rescue stick/fishing pole/whatever you can think off - the radio is likely to cut out and leave you stranded in the open water.
Most importantly of all, have fun!