the reason i said a standard AM radio would have problems is that the IF is 455khz, and if you re-tune the oscillator to shift the receiver's range down to 455khz, you will have your original signal bleeding through the mixer into the IF, and you will have two signals in the passband 2khz apart. there are reasons standard AM radios don't go below 500khz, and LW/AM/SW radios (single conversion) leave out the range between 400-500khz.
plus, just soldering a cap in isn't going to shift the reception range. you have to change both the RF stage tuning, AND the local oscillator frequency. when the radio is tuned to 457khz, the oscillator frequency will be 912khz, and the difference between the received signal and the oscillator will be 455khz (the IF frequency. but since those mixers are so cheaply made, the received signal of 457khz will be passed through also, giving a 2khz whine in the receiver.