Here is a site that sells time serts not sure if they price or site is any good just pictures to ref.
+ TIME-SERTŪ SPARK PLUG THREAD REPAIR KITS + spark plug stripped thread repair kits for spark plug problems, thread stripped spark plug repairs in damaged threads in spark plug hole stripped out approved thread repair, aluminum head spark plug repair
++ TIME-SERT SPARKPLUG Standard Repair ++
These can be done with the head on but i would not. best to get it rigth from the get since its got to last a long time. Use plenty of antiseize on your plugs once complete.
Also if you mess up the time sert the head is trash. So thread chase tap may be better. But you will need to go inside out since the threads will not start on the other side.
Never ever use a heli coil to fix bad threads if a time sert will do the Job these never come out and get screwed up they are usally better than the original threads. Heli coils will crap out the first time the plug is removed usally.