If you have ever taken a coil apart, you will find that the secondary winding has many thousands of turns of "hair wire", so thin that the slightest tension will break it. The layers must be absolutely even, with a layer of very thin "condenser" insulating paper (or plastic) between each layer. In short, you must have an expensive solenoid coil winding machine. There are a few such machines in the USA, but they are not used for our spark coils.
The coils now being made come from China. The reliability problems they are having would demand more interaction between the US suppliers/importers and the Chinese manufacturers. This doesn't seem to be happening.
I also have repaired Modelectric and AeroSpark coils. Vibration breaks the internal wires where they are soldered to the lugs. Also, they are held together with bee's wax, which doesn't hold up to vibration. After my repairs, I seal the housing together with epoxy to prevent further failures.
Floyd