If you want to stick with NGK, they offer an iridium plug in the same heat range, BPR6EIX, that is readily available. Granted it's a resistor plug, but it uses a fine-wire iridium-electrode that delivers a hotter more concentrated spark. Iridium plugs are used in many modern engines for their durability, they're a 100,000 mile plug, and their performance advantages. Check them out. I use them in my 807 engine and have no complaints.
Below is an extract from a technical article written by Gordon Jennings in a 1977 issue of Cycle Magazine. At that time Iridium was not available, but the advantages of a fine-wire electrode still apply.
"Platinum and gold-palladium alloys can survive the combustion chamber environment as very small wires, and in that rests their great advantage. Electrons leap away from the tip of a small-diameter, sharp-edged wire far more willingly than from one that's fatter and rounded. So the fine-wire plug requires less voltage to form a spark than one with conventional electrodes, and the difference becomes increasingly biased in the former's favor as hours in service accumulate and erosion blunts the iron-alloy electrodes. There are, of course, drawbacks with precious-metal plugs: they are more expensive, and they are very sensitive to excessive ignition advance."
