The following diagram is the schematic diagram of electric guitar effect: Electro Harmonix (EH) Big Muff Pi.
The EH Big Muff Pi would probably be better making the change by a modern input-jack power and a DPDT bypass switch. The types of transistors and diodes are unknown. It is likely that any high gain NPN transistor and diode 1N914 will work. Coupling capacitors marked with * have been reported at its best if it has changed to 0.1uF and the capacitors marked with ** changed to 1.0UF. The original transistors were labeled SPT 87-103, and diodes of origin have been marked or 525GY or 523GY (difficult to read.)
Download Electro Harmonix Big Muff PI guitar effect circuit diagram in PDF: