[MartijnBraam]

Author here, the best solution I think would be just ripping the temperature sensor from the battery pack and soldering it to the contact in the microphone instead, then you can just pop in regular rechargable batteries and it'll work. No need to add lipos to this.

[CarVac]

The thermistor looks like a Semitec ###NT series, if you want the exact one.

103NT H34G, I suppose?

[MartijnBraam]

Oh that looks incredibly similar, thanks for the tip!