It looks like the external mic connector SJ2-35464a is obsolete.
I tried to find it but ony found some, quite expensive, on a Chinese website. Maybe someone knows where to find some. I also tried to look into other models. The trick used seems to be a TRRS socket where the second ring makes contact to GND via an extra contact or the sleeve contact. This switches a small analog switch. In theorie an other TRRS socket could do the trick but the pins on other models do not match.
The closest thing I could find looks like this. Unfortunately I do not own the audiomoth v1.2 to prove if this could work.
3 pins seem to match, two others are close and need a blob of solder or a short piece of wire. If I am not mistaken the SJ2 connector already had one ring-2 pad already connected to GND. In the P327 version this is the Sleeve contact. I am not sure if only one or both the Tip contacts were used on the SJ2 connector, so maybe a piece of wire is needed.
An alternative could be a small PCB like this. Again I am not sure where the tip contact should go, one of the red wires should be enough, maybe either pad is good. Please keep wires short or uses good shielding.
I do hope someone has a good source for the right type of socket.