So the spark plugs are replaced, and the misfire appears fixed - so yay! But the Speed Four isn't yet back on the road.
Long story short, removing the tank for the spark plug replacement (finally) killed the stock fuel line fittings. It's a known issue that the stock ones are junk. I bought after-market ones, and I received them yesterday. Aaron says they're definitely more robust than the stock ones. We're waiting on a couple of other things for the fuel line fittings replacement (and also that gasket to keep the spark plug well from filling up with oil again), and then the Triumph will be back on the road.