The Merlin XII of the Mk.II used a Coffman cartridge starter, rather than electric start. Some magnetos use an “exciter” (I think that’s the term, in US, anyway) for starting, which boosts the amperage, causing a “hotter” spark. This requires an electrical source besides that generated by the magneto itself, which may mean it wouldn’t work with the Mk.II’s starting system. I can’t speak for other Merlin varieties offhand. (I’m also going on old memory, so some of the details here may be wrong!)

bob