Tremolo effect produce an amplitude modulated signal. Unlike vibrato effect that produce frequency modulated, this tremolo effect will produce the same effect as quickly turning up-ad-down your volume control repeatedly. Here is the schematic diagram of the circuit:

Analog Tremolo Guitar Sound Effect circuit schematic

Q1 is general small signal audio transistor, you can use almost any type of NPN small signal transistors. MFC3304P is a voltage controlled amplifier  IC with negative control, it might be difficult to find these days,  but I think you can replace with general OTA (operational transconductance amplifier), with the appropriate biasing circuit off course. Just bias the current control of an OTA through a resistor, and add the C4 modulator signal at the control pin of the OTA. [Circuit's schematic diagram source:]