Any thread repair without removing the head has to be taken as a one stop attempt to reclaim the ally, but it is possible as previously said by others.
I cross-threaded my one of the heads on my CX500 many years ago and the plug literally fired-out when the bike started.
I took a plug and hacksawed a slot down the threaded shank, cleaned all the swarf off and used it to act as a taper-thread, with plenty of grease.
Yes, it worked and I did thousands more miles with that re-cut thread, but I always had to 'oil' the new plug on that head when i replaced plugs at service time.
The thread was 'tight' and I didn't have the funds back then to do the job correctly or worse-pay someone else to (us engineers are generally resourceful but certainly not wealthy)