The impedance match may be achievable passively and with decent efficiency by using an impedance-matching transformer.
First of all, I suspect that the 5 ohm impedance refers to the source impedance of the microphones, not the input impedance of the mic preamps in your helicopter's instrumentation panel. If I were to guess, I would say that this impedance is probably in the hundreds of ohms, or even a thousand something.
Let's just go with 1000 as an example.
You should also find out the impedance that the high impedance microphone expects from its preamplifier. Let's say for the sake of the example that the mic works well if it faces a preamp with a 100,000 ohms impedance.
So, in this example, what we need is a transformer which makes 1000 ohms look like 100,000. It will be a step-down transformer, and the turns ratio is the square root of the impedance ratio. 100,000 / 1000 = 100, and the root of that is 10. So 10:1 turns ratio would do the job.