One note.
There's a difference between "chasing" and cutting the threads.
The last thing you'd want to do when dealing with a partly stripped or damaged thread is remove more metal through filing, or by using a cutting die. Those would remove metal, but do nothing about raising the peaks so they're high enough to to engage the headset cup or locknut.
The objective is to flow metal and reform the peaks in a process more similar to rolling threads than to cutting them. This is what chasing is about. Of course this only works if there's enough metal there to salvage.
The tool of choice for this is a decent condition steel threaded upper cup. Oil it well and work it down the thread by degrees, backing off as it jams, and hopefully making progress three steps forward and two back. Keep it well oiled, and remove completely and flush to clean off any fine metal shards which may accumulate.
The chasing process isn't magic, and can't add metal, but if the old thread was distorted, it can flow metal from the sides of the thread form back to the peak, raising it back to profile.
Otherwise, you need to add material via brazing and cut fresh.
There is another option, there's enough thread to hold slightly. Replace the quill stem with a threadless adapter stem and use a threadless extension. The upper part of the adapter needs to be jammed down against the locknut to hold everything together. Even if completely stripped the headset can be adjusted by setting the stem tight enough to hold, but no full tight. Then it can be tapped down to preload the headset, and locked in place.
Or you might be able to finesse the job by using an
adjustable collar under the adapter.
What's best depends on the condition and worth of the fork, and the cost of replacement vs. other parts needed for a change. But I suggest you start with the simplest, cheapest alternative, and scrounge up a steel cup, and give chasing a chance.