Converting a non-electret condenser microphone circuit to electret?
The 2N5551 and its components are an oscillator and a charge pump that takes the 12 Vdc from the zener supply and boosts it to 40-60 Vdc.  You are correct, none of this is necessary with an electret cartridge; 12 V is more than enough to excite the capsule.  Keep R10, D4, and C6.  Note that D1 and D2 are backwards on the schematic.

The mic needs a bias current.  Connect the left side of R13 directly to the zener diode +12 V, and reduce the value to around 15K-22K.

Using the numbers on the datasheet:

Mic current = 0.5 mA

Mic voltage = 1.5 V

Source voltage = 12 V

Voltage across the R13 bias resistor = (12 - 1.5) = 10.5 V

R13 = E / I = 10.5V / 0.0005 A = 21,000 ohms