The original microphone for KX-3, Elecraft MH-3, is insanely expensive. What changes would be necessary to convert a cheaper speaker/microphone for a FM handheld transceiver? Is there any ready-to-buy adaptor?
The Elecraft rigs have flexible interfaces for alternate microphone settings. However, there is only the single TRRS plug for a Microphone on the KX3. You should be able to wire up your own microphone to the diagram I have included here (extracted from the User Guide).