The devil is in the details. If the "mechanic" started the injector and cross-threaded it but did not force it past two or three threads............you'll just need to clean up the three threads with a proper tap. This is not an easy task, however, because the tap wishes to start on the crossed threads and it takes a skilled machinist to force the tap to start on proper threads.

If the entire set of threads is destroyed, you're looking at some major work. You'll need to drill it out for a timesert or a Helicoil. This is really machine shop work and the results on the engine with a hand drill might not be satisfactory. Furthermore, the prechamber will fill with metal chips and you'll probably need to pull it to be sure you've got all the chips out of the holes.