Quick update on this.
The modem fits fine in the phone, it does require some re-routing of the antenna cables though. I haven’t actually done this yet, but it all seems to be possible.
The modem works as a drop in replacement for texts and data. For calls initiating a call works fine, but the audio is distorted and has lots of static. I suspect it’s a codec issue, which is described in the device tree.
I have no idea how to fix the audio though. So unfortunately we are stuck