A 1N5711 or a REAL Germanium (1N34 - 1N60) would be somewhat easier to mount. Any small signal germanium or schottky diode would work as well. If one pulls the base of the 'switch' transistor below about.5 volts, the transistor is in cut off, non conducting. There is however an easier method that doesn't require cutting traces and 'hanging parts on the display... and that is to replace Q1 (9013) with a BS170 or a VN2222, VNK10 or any other n channel small signal enhancement type Mosfet. The circuit looks like it was designed for a Mosfet and someone 'goofed' and put a bi-polar device in it's place.
IMO and direct experience.
Doc