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?
In the end I decided to build a simple adapter. Most handheld microphones available at cheap prices have two-jack "Kenwood compatible" interface. They have 3.5mm TRS jack and 2.5mm TRS jack.
The sleeve on 2.5mm jack provides ground contact. PTT contact is on 3.5mm jack sleeve and microphone contact is on 3.5mm ring.
I bought two jack plugs, soldered the respective contacts on a single 3.5mm jack and, while the plugs were on the handheld microphone connector, fixed everything in a chunk of sugru.
Everything works fine. It is necessary to choose the right microphone. Wouxun microphone purchased in a local ham radio store has nice modulation, while "Baofeng" mike purchased on eBay is noisy and the modulation is unpleasant.