Luddite I carry a spare switch Intermotor brand No 50610 but cross-referenced to other manufacturers. Rover have two references 589740 or RTC5930, Landrover are 589470 and lucas SOB812. Also lots of other manufacturers.
I have the Haynes manual for the SD1 ( Mog is 77 with SD1 engine and g/b ) and that shows wiring to and from switch. Hope this is some help.
From deep deep downunder