there is no way to install a long coil like a Johnson because of the "V". As for shorter coils, the shape of the armature core being rectangular would not accommodate any modern coil that I know of
I agree. But what I suggest is machining that core so you can fit another coil in. The issue with the Evinrude mag plate is the raised wall that was used to hold the tar in. I don't think it is needed structurally.-- And it prevents installation of a coil from one side of the heels to the other.
If you look at an Evinrude Speeditwin coil,(and I'm only using this as an example) the laminate has a step on each side and the cores the coil mounts to has a step. One side is up and the other down, and the coil only has a machine screw that fastens the core to the heel. There is no interlocking of the laminates. So in theory, if a Speeditwin coil would fit in the space (yes, I know it is for twin cylinders) between the heels, and if you cut out the middle of the core, machined a step on each side to match the Speeditwin's steps and mounted it like it is on a Speeditwin mag plate, it should produce spark.
This thread has encouraged me to seek a bad mag plate and experiment! I've done 4 other mag modifications installing other style coils with great success. I put a later Speeditiwn coil in a model R Fastwin, a Johnson S/V-45 coil in a 30 cu in Caille and in a 1916 Koban, and 2 OMC coils in a Caille Master 5-Speed.