My previous comments on the head appearing normal were taking into consideration the commonly poor quality that the HT engines are known for. While I agree that such a flaw would not be acceptable in a higher quality engine such as a Honda, Tanaka, Mitsubishi, ect. they are part of the game with HT's.
If you elect to use a longer reach spark plug be very sure that all the carbon has been removed from the threads in the head before installing the new plug. Also, absolutely all of the threads on the spark plug itself must be covered by the head to prevent carbon build up on them. If any carbon deposits are allowed to accumulate on the spark plug threads it will ruin the soft aluminum threads in the head when the spark plug is removed at a later date.