It's not clear what you are trying to accomplish. Normally you can connect an antenna and you'll pick up signals from AM stations in your area. If you are trying to repair the radio, perhaps you need a strong signal as a starting point. For that, you can use a signal generator. (Or make one...making a 1 MHz oscillator isn't that hard. Making a stable one is a little harder, but perhaps not necessary for testing).
I did a little Web research, to ballpark some numbers. These numbers apply to "shortwave" but that's close enough to an AM receiver (which is probably not even as sensitive as a shortwave receiver). A good signal (S9) represents 50 uV at the antenna input. Signal meters tend to go up to +60 dB over that. That would be 50 mV at the antenna. I'm sure receivers are subjected to even more than that. Your 1920's set will be using vacuum tubes, and they will take quite a bit more abuse than that.
I'll mention in passing that you should know if and how the chassis is grounded (or isolated by a transformer). Basic personal safety is important, especially if any of your test equipment is also grounded.
As a bonus comment, I'll mention that a radio with a 455 kHz IF will have an oscillator of its own, operating 455 kHz away from the receiving frequency. Sometimes you can use another radio to produce a signal in the one you are working on.