Can you substitute a silicon transistor in place of germanium?

It can be done if absolutely necessary, but usually the circuit will not function properly unless totally redesigned to compensate for the differences between the components. This is rarely worth the trouble unless a germanium substitute is not available and the circuit MUST be repaired at any cost.

So unless you know exactly what you are doing and how to design transistor circuitry from scratch (after reverse engineering the old circuit), consider such substitutions to be impossible.