For independent motor control of one stick you will need to buy 2 separate esc units with the correct amp ability a for your motors and a mixer unit.
Or you can splash out on ACTions p94 or its smaller 10amp brother, which is an all in one unit for just that purpose.
Either option is expensive with the p94 more so then the first suggestion, but well worth it's money in its ability a but honestly lost on a bait boat!
You will also I expect have to buy a new remote for your project as I do t know your ability a to mod what you have to work with new electronics, it's unlikely you could get the current transmitter to work as I expect the receiver is part of the machines circuit board, so you will have to cough up a bit more on a new transmitter, there are cheap ones out there like the flysky from hobby King, which is 5 channels, search around for a bargain.
I can only assume you will need several servos for other functions.
Honestly it might just be worth buying a new one.
Or another cheaper option.... One I use and doesn't add headache to my fishing....sling shot the bait in!