I can recommend 3 options,
Replace with known good OEM coil, Been there done that..
Replace with OMC coils, this swap has been covered and can be found in a previous Outboarder, I recall Jim Moffatt did the article and I supplied some photos.. Been there done that, got pics to prove it
Replace with a Speedifour coil.. They are close… I just did one I think it was the Speedifour coils…
I have the last conversion on my bench, makes good spark.
I can offer to convert for you, the issue being, I will shortly have no power head and flywheel to check for spark, as I have sold my Mastertwins. They are still here but spoken for.
Richard, I think you’re thinking of a different style Eisemann magneto.
Mine is like the one in the photo, from this thread….
https://aomci.org/forums/topic/neptune-1940-6hp-coil/
Prepare to be boarded!