Ray,
I have heard of oversize cylinders, uprated crankshafts, and even conversion into a v-twin format; but not the addition of a electric starter. Probably because there is no way to arrange the drive without chopping through the engine castings, which would entail some serious engineering. Since the Mark series is one of the easiest bikes of its era to start (one in good fettle can be started by hand applied to the kick starter), no one has apparently though it worth the trouble to arrange such a system.
Years ago one or more folk installed a Panhard opposed twin into a postwar Douglas (Dragonfly I think) that might have retained the car's electric starting system. But I think the purpose of the exercise was to get a bigger engine.
-Doug