Your "replacement" transistor would not work.
The P423 transistor has the following characteristics:
- Material: Germanium
- Transition Frequency (ft): 100 MHz
- Collector Capacitance (Cc): 10 pF
- Forward Current Transfer Ratio (hFE), MIN: 24
The BF324 from the schematic:
- Material: Silicon
- Transition Frequency (ft): 350 MHz
- Collector Capacitance (Cc): O.1 pF
- Forward Current Transfer Ratio (hFE), MIN: 25
As you can see, not only are they made of a different material which makes their base-emitter bias voltage different, but more importantly they have significantly different maximum frequency.
The transition frequency is the frequency at which a transistor has a gain of 1, meaning the frequency at which it stops acting as an amplifier, and after which it actually reduces the input signal.
Another important factor is a transistor's own inter-electrode (between its own electrodes/pins) capacitance, which can significantly weaken a high-frequency signal.
The original, BF324 has only 0.1pF collector capacitance, while your replacement has 10pF, which is 100 times higher! That will definitely make a difference at very high frequencies.
Your replacement transistor can't amplify FM signals because their frequency is too high for it, but the BF324 definitely can.
In high-frequency circuits, you should always check to see if the replacement transistor's maximum supported operating frequency is as high as, or higher than, the original, or at least close to it.