In the directory containing your MicroPython external C modules try:
git clone https://github.com/wasp-os/bma42x-upy
cd bma42x-upy
git submodule update --init
After updating the submodules see BMA423-Sensor-API/README.md
for more
information about the driver.
Thanks to Aaron Christophel (ATCnetz) for doing the initial reverse engineering. bma42x-upy doesn't contain any code from Aaron but his work was hugely important in helping us understand how the hardware works.