The OEM plug caps price is a rip-off.I've bench-tested these NGK SD05FM Reds(They do Black as well)
http://www.ebay.co.uk/itm/Spark-Plug...wAAOSw7NNT6WRf
They seem ok but have not road tested them.
However the originals can be deconstructed using the correct size Flat Blade screw driver.The Brass insert is soft so I re-dressed it after checking and cleaning using a junior hacksaw and a flat file just to make the blade groove better.
The Resistance was just below 10 Kohms.The NGKs only have a 5kohm ceramic resistor but I don't think that will affect their performance.I couldn't find a Data sheet on the OEMs to check what their original resistance should be.
I'm using,"R" rated Spark plugs which means they already have a resistor in them.
If you can check yours for me and they come out around 9K ohms I'll know that's the original spec.
Spray some WD40 or similar inside the plug cap before deconstruction to help loosen.This can be cleaned off later with Carb/Brake cleaner.
Be careful if you choose to deconstruct as there's a tiny little brass dot on top of the spring inside.Easy to miss/lose.Cap would most likely be ok without it but if possible best kept.
I made a new little dust seal from some old motorcycle inner tube I have so nothing could get down the plug hole.If I used an NGK straight one on the front I'd do the same but make a second larger circle to go over the hole as the front one is more open to the elements.