!@#$ it! I just typed a long response and the website/computer froze up. lost it…OK, one more time….
Always start with the simple stuff…Check the coil grounds and pack ground using an ohm meter on the low scale, reading should be as close to zero ohms as possible. Next, it is a good idea, to eliminate the stop circuit from the system. Yes, not likely that a faulty stop switch would affect just one cylinder, but never say never.
Next, I would inspect the ignition wiring and disconnect the five pin connector coming out from under the flywheel to the pack. Any signs of melted/chafed wiring, perhaps a botched previous wiring repair? It might fight you, but you can wrestle it apart carefully. Have a look at the pins and sockets, any misaligned/partially pushed out of the connectors? Do not spray lube/oil/grease inside the connector before reconnecting the two halves, this could cause a partial short that will cause trouble down the road. Use rubbing alcohol inside the two halves to easy connections, this will evaporate and cause no problems.
Next, pull off the coil primary leads from the coils. These connections are a big trouble area, especially in salt water. The connections get loose, rusted causing ignition headaches. I would clean up the terminals, the crimp down the connectors slightly to ensure a tight fit/good connection. Go ahead and use grease on these terminals, won’t cause problems here. Now, I would swap the coil primary leads. Put the top primary lead on the bottom coil, and the bottom on the top coil.
Check for spark again.
If #1 still does not fire, then you have some sort of coil problem, perhaps a messed up secondary wire/boot/terminals.
If #2 does not fire with the leads swapped, then you have a pack or sensor problem….
Let me know what you find…D

I’ve replaced kill switches on a couple of 1991 15 HP Johnsons with generic ones at $10 or less after seeing the $100 price tags on the OMCs. I mounted the replacement on the front panel because it wouldn’t mount into the tiller handle. Most of these replacements have normally closed and normally open contacts so they will work with magnetos or electronic ignitions.
Dave

The Seachoice or Seadog inthe link below should work:

https://www.iboats.com/shop/electrical/marine-switches-battery-ignition-rocker-and-toggle-switches/emergency-boat-kill-switches-and-lanyards.html