Only a quick idea but my guess is your on "10mm" plug sizes at the moment (the smaller,more common of the two,could you not just go to the 14mm older size on just that cylinder?
Chances are the original hole+wear will be very close to the new size hole for the thread that you need and you can simply tap to that plug size.
I see no reason why it won't work,it is a little bit bodgerish but if you were that worried you could re drill/tap the rest to suit.
Not sure where you stand with the debris from doing it,ie swarf,my guess would be pulling the plugs and turning it over to throw the crap out could be an option as a magnet won't be much use really as non ferrous.