These transistors are prone to failure.
In most cases, they can be replaced with modern general-purpose silicon bipolar transistors with no modification to the circuit.
Reasonable silicon replacements for small-signal NPN and PNP germanium transistors are 2N3904 and 2N3906, respectively.
When transistors appear in pairs in the original circuit (differential amplifier, Schmitt trigger) change the pair, not just one transistor.
When a germanium transistor is used with its base-emitter junction in parallel with a germanium tunnel diode, you cannot simply replace the germanium transistor with silicon. It will not work because the "high state" voltage across the tunnel diode will not be enough to turn on the transistor. If a germanium transistor in this type of circuit fails, you either have to find a working replacement germanium transistor or modify the circuit to work with a silicon transistor.