Schottky diode as AM detector?
If 2AP9 is similar to 1N34A, you can surely still find those, although I'm not sure about availability in China (your location).

As for using Schottkys as substitute, 1N5711 (a ~0.2V [@ 25C] UHF Schottky) does work as substitute in some designs; 1N5711 has only 2pF capacitance. YMMV if it will actually work in your radio, but it seems to have a better chance than 1N5819.

There are some research papers on using (CMOS-fabricated) Schottkys as ultrawide-band AM detectors: "The input and output matching of the detector is better than -10 dB from 0-10.3 GHz". I'm guessing that a worry might be getting too much band and thus possibly HF noise when using a Schottky instead of a Ge diode for an AM radio in the classic AM radio band.