I don't see a rudder which suggests the steering is controlled through the twin motors. Probably not any replacement parts available as it's more than likely an all-in-one unit for electronics and probably disposable.
Did you verify the voltage on the battery with a voltmeter?
If the battery is still taking a charge then that's not your problem... use a voltmeter to confirm the battery reads 8.4V to indicate a full charge... if not at 8.4V then the battery needs replacement.
Are you getting steering response? If so, then check the motor by applying voltage directly...