When a friend was investigating a fuel leak located at a fuel injector (IH diesel truck engine), he discovered a motor monkey had previously cross-threaded the line nut onto the top of the injector, so he proceeded to remove the injector (the line nut was good).
He went to a nearby heavy truck dealer and bought a remanufactured injector, copper crush seal, and O-rings for the fuel return fitting. The reman injector was only about $46 plus a core charge.
A problem developed, as the bottom portion of the injector separated from the larger, upper portion. The part was down in a restricted recess about 2" deep. Since the upper injector portion had a very coarse thread, but the bottom portion had a very fine thread, it would be very unlikely that the parts could be reassembled in the engine.
We speculated that real truck mechanics probably have a special tool with the proper thread to screw into the lower portion, which could then be withdrawn with a puller or slide hammer.
The conclusion was to sacrifice the fuctup injector, as it only has a value of $13 as a core charge when purchasing a remanufactured injector.
Chucked the injector upper portion in the mini lathe 3-jaw and cut away the coarse thread lickitysplit.. in a shower of hot chips.
We had seen that the bottom portion could've been heavily coated with carbon and potentially difficult to remove.. but it wasn't. He ended up with a removal tool for future use, which could be adapted for slide hammer use if needed to gently "bump out" another part if the problem arises.
The worst-case of needing to pull a head off will likely cause your trip to be postponed.