Tercel Spark Plug Threads blown out

I have a 1992 Toyota Tercel; it uses spark plugs NGK RN11YC4. The plug threads for cylinder #1 stripped out, and I repaired them with a standard rethread kit. After 60,000 miles, the repair threads blew out. My mechanic and a machine shop tell me there is no oversize repair kit, and I should replace the head.

The original plugs are 14mm. There appears to be space to tap out the threads to 18mm. Is there an 18mm equivalent to the NGK RN11YC4?

Thanks