From what I know... the "esc" is built into the receiver..... So you would be getting a new receiver.
I have heard that there is a problem with some receivers.
You are dealing with inexpensive electronics. So go figure... ie.... snafu's...
I have two 50's... just so I could beta test them against each other...
I have gotten motors with the wires switched and they spun backwards... and
then switched the wires not switching the diode... and burnt up motors and a receiver
catching fire...
Everything on the "bird" is controlled from the receiver... so you may have a bum receiver.
Hopefully it will work out for you. Obviously... if you drop a new receiver in... then it may
be a loose wire in the transmitter... then it's almost worth buying a second heli that comes
with the transmitter... and another whole bird of spare parts or beta bird.
On more expensive heli's the gyro... the esc.. and the receiver are separate.
Thusly... on the 50's et al... that's a lot of stuff in one tiny little box.