I recently had to repair a spark plug hole that I stripped out on one of my CT90's and ended up using a Time-Sert threaded insert. While not the cheapest way to go, I think it is the best thread repair out there. I did a post at my CT90 blog where I share the process you follow to install a Time-Sert insert and I have included a link to that post below in case anyone else was thinking about using a Time-Sert and wanted to see what was involved.
https://ct90restoration.blogspot.com/2018/06/repairing-damaged-spark-plug-threads.html
Thanks,
Mike
https://ct90restoration.blogspot.com/2018/06/repairing-damaged-spark-plug-threads.html
Thanks,
Mike