Advice please on ESC/motor

Hi, hoping that you learned folk can help me with some advice.
I have a RC boat that belonged to my son and has sat in the garage for 20yrs. My grandson now has an eye for it and I am trying to bring it back to life.

New batteries got me to the point that the steering servo works and I have tested the motor works by applying power directly. 
The problem I have is that I cannot get the motor to run on command from the the RController. I have swapped the channels (swapping the leads on the receiver) and both channels can drive the steering servo. Neither can drive the motor.

So my belief is that there is a problem with the ESC. Not a clue what I need by way of replacement though. What I do know is that it is a 540 motor, has a BEC to the receiver. The ESC is marked 15A and it has a 7.2v battery as standard (although I would prefer to fit a 9.6v if possible). 

Am I better off buying a motor and ESC as a kit?

Any advice most gratefully received.

Thanks

Most 540/550 motors are rated 15 amp constant current, however they do require 20 amps on inital start (burst current)

In this case if its the orginal motor and 20 years old.. replace it! (odds are the esc has dug its grave)

Then buy a new ESC....

I Recommend This One (Click)
If you can find a 20 amp version that would be better and a lil cheaper however there is nothing wrong with a slightly bigger ESC as it saves issues on buring out the ESC should you stall the prop, i Do NOT recommend replacing it with another 15amp esc.. you will not get the same stall protection and always risk the motor burning out the esc on burst currentlys when you throttle)

A Decent 550 (not a nasty £5 cheap thing)

Power to the ESC can be from as little as 3V up to 12V so your choice of battery is vast, Including Lipos!

once installed Mtronics escs have a throttle start and stop program switch ( or the older ones did) but they are generally fool proof for instalation.

This should resolve most of your issues.... also remeber to clean & grease up the prop shaft and tube (friction in the shaft = more amp usage = shorter run times)

Really appreciate the advice Roadrunner.
I will procure the necessary and hopefully have it back on the pond soon.
Many thanks.