If you are sure it’s an electrical problem, I’d double check the Bakelite brush holders with a magnifying glass for any signs of voltage leaks or carbon tracks before condemning the coil. The HS-10 I was working on would run intermittently on two cylinders and then drop to one. I was starting to suspect a bad coil to but switching the internal parts around helped to figure out the problem. It was a tiny hole in one of the brush holders allowing the current to jump to ground thru one of the hold down screws.
Before I dug it out, the hole was almost invisible but I could see the carbon track after a good cleaning.
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