I've used Champion J-8, AC 45, AC R45, and Autolite 303 spark plugs in my CA and other 6 volt tractors and vehicles on the farm. Autolites seem to run the best in my CA and they sure make the B we bought this spring run sweet (haven't tried them in the WD45 yet). My old Willys jeep (with 12 volt conversion) runs good with them too - it also called for that same spark plug. Champions seem to run the worst (and don't last long). AC's did pretty well but I think the AC R45 is no longer available (think the substitute is an AC R46 now) and the AC 45 has been out of production for a while.
I've been told an Autolite 295 (non-resistor) is for a magneto ignition tractor (but can be used with battery ignition) and an Autolite 303 (resistor-type) is for battery ignition. Your CA probably has battery ignition - you could use fiber-core plug wires if you needed to. ONLY use copper-core wires for magnetos and it's probably a good idea to use them on any 6 volt system (I use copper-core on my 6 volt stuff - I get them from NAPA here. The local CarQuest doesn't carry them). 12 volt converted tractors seem to use the fiber-core wires with no problem.
I've also had a bad Champion and a bad AC plug straight out of the box. When you pull the plug wire slightly off you're increasing the voltage potential before the plug fires (longer effective gap) and so it fires - had the same problem on an air-cooled engine once that turned out to be burned points. The spark plug must be faulty in the inside somewhere.