It is possible to remove the remaining portion of the plug, but I would strongly recommend having it done professionally, the potential for cylinder head damage, foreign bodies entering combustion chamber etc. is extremely high. The money you invest will be by far outweighed by the incurred cost of head removal/replacement. Most cylinder head specialists or competent engineers will be able to provide this service.
Incidentally, using penetrating oil would most likely NOT have helped the situation,as the siezed area is(in general) near the combustion chamber end of the thread. Having said that, in future I would recommend using a nickel based anti sieze compound (sparingly) on the thread of new spark plugs. Be careful not to contaminate the porcelain area, as this compound is electrically conductive.
BTW.. the possibility still remains that the siezed thread was caused by coolant entering the combustion chamber, causing corrosion, so the head may need to be removed regardless....good luck!! :)
pomandrew
April 2009
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