im not gonna say its impossible, but i see alotta extra work in trying to do it.
the only way i can see that working is you for one would completeley do away with powering the forward/reverse part of the board that actually send power to the motor, then you would also do away with powering the stock esc directly from the battery pack, the new esc would need to be powered directly from the battery, it would then need to power the reciever part of the stock esc, and also the steering function of the esc, if you were to buy a hobby grade esc, it would have 2-3 wires for the motor to hook to, you would only use 2 of them, it would have 2 wires to connect directly to the batter wires, it would then have 3 small wires, these wires are what normally would plug into the hobby style reciever and when plugged up these 3 wires would do 2 things, they would 1 power the reciever, which also supplies the power to the steering servo 2 they also when the reciever gets a signal they power one function or the other, forward/reverse.
that being said, you would have to by-pass alot of circuits in the original esc to make it do nothing but work the reciever,give signal to the esc for forward and reverse, and work the steering im not gonna say it cant be done, but you would basically be re-engineering the board and taking on alotta headache, even then im not sure it would work, and you would still need to fix whatever relay/capacitor/transistor or whatever is wrong with the original esc, since its probaly not the part that gives power to the motor thats bad, but most likeley the part of the esc that gets the signal to supply power to the other part of things, steering servos, and the recievers are not very expensive if you buy a radio used on ebay, and you can get 7.2v esc's pretty cheap also, especially brushed. you would be light years away just going all 100% hobby grade if your gonna swap esc's, not to mention, hobby grade digital proportional is alot better than the nikko/radio shack digital proportional, alot smoother/quicker/or even slower if ya like it that way