Well I understand that the spark plug does not damage the thread of the cylinder head. If this is so only a thread rectifier happens to clean it when you put the spark plug, it must be well tight but you are in problem ... if what is neighborhood was the thread, let me know
The first thing is to get the thread from the spark plug. A good welder can do this without having to disassemble the cylinder head, and then before anything tests putting the new spark plug if everything enters well if not, testing reviewing the thread with a male, and tests again, if you do not work you take out the cylinder head to send it to the rectifier to make the thread but never die so much.