Sorry, I must have missed the thread when it was started.
The BC264 is a standard edition J-FET, suitable as impedance converter in condenser mics. You can conveniently substitute it with a 2N3819 which Neumann uses and used in almost all of their FET mics.
Besides, I do not think that the low pass response in your mic is caused by a defective FET.
Usually, severe high end attenuation and low output points to a defective capacitor and/or that second transistor which may be there for biasing. I would start by replacing it.