Turner 254 Replacment Element
You could probably successfully use one of the modern cheap electret elements to drive your FT-101. The problem is you need DC to run the mic. Presumably not easily available at the mic plug, it would require modifying the radio to put DC from the radio onto the audio line and then block the DC inside the radio with a cap. Or put a battery in the base of the microphone. That could work if you have a switch contact available to just let DC current flow to the mic when the PTT is pressed down to save battery. The FT 101 works quite excellent with a Shure 444 microphone, or the possibly cheaper to acquire 450 with the switch in the hi Z position on the bottom. That is, the Shure mic is hi Z like 50,000 ohms but not the megohms hi Z of a crystal or ceramic mic. I am pretty sure that the original Yaesu mics were low impedance mics with a stepup transformer inside them. A purely hi Z crystal mic would probably be a better match to the HT 40 tube rig than the solid state Yaesu.