Did a replacement of my S42 Driver with one from Lexel. it’s a great driver and came with the Narsil ramping FW (same/similar to D4). With some tweaks explained in Lexel’s Thread and in some of the “what did you mod today” posts I was able to continue using the original USB charging board, The S42 is now what it should have been all along!
FWIW, Lexel indicates that his S42 boards are 20.5mm, I didn’t measure mine.
The MtnElectronics driver should work but I don’t know whether illuminated side button and USB charging can be made available, best to contact MtnElec directly, they have great comms.