Firstly, put in the correct plug. That nkg you mentioned is a cold plug with a short reach and a different type of sealing ring. It would have threaded in but probably not be sealing properly. If the threads are stripped, replace the head. Any thing else you do to a aluminum head will only be temporary. Aluminum heads will not hold any sort of helicoil insert very long due to different heat expansion rates between the head and the coils.
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