Fred, I'm not really the one to ask.
From what I have read, the M5 and M6 are the same except for the wiring.
For the M6 that I got from Richie, I have new electrics (solenoid, governor and interuptor switch), I'm making up a new harness using the original and the wiring diagram as a guide, and flushing and refilling. I have the correct carb which a friend with extensive carb experience is rebuilding for me.
My own experience long ago and all I have read tells me that should be all that is required as these units are fairly trouble free.
As for the fluid coupling, it will get drained and refilled as well. The clutch is in good shape and dry.
Hopefully, the unit will operate well and only need minor adjustments
Richies M6 upon arrival
As of yesterday on the bench so I can install the new solenoid, governor and interuptor switch.