I have read the twin post with interest, not knowing anything about them. I note many coil problems and suggest the use of the original coil (the primary winding would probably be OK) and external connection of an old Honda 125 twin motor cycle coil in series with the original coil primary. The Honda coil has one primary winding with both ends terminated in flying leads, not connected to earth and two secondary high tension leads. Primary coil resistance is close to one ohm, close to the resistance of the primary for a series 80 coil that I measured. The Honda was six volt points ignition.
Another trick for low coil output is to close the spark plug gap down to 0.3 mm or even less, the voltage required to jump the smaller gap will now also be less. I have run an engine with the the plug gap at 0.1 mm and it still ran fine to get me home (about 50Km) after the coil would not fire the correct gap. Obviously had to replace the coil eventually. A large plug gap as used in modern cars is desireable to fire lean mixtures reliably, the rich mixture used in mowers will always be fired by any spark, even a small one. In my experience if the spark is OK the mower will start and run even if the carby is not quite right.