It's hard to tell from the picture whether the threads are damaged or just clogged with aluminum debris from an aluminum headset part. If the latter, simply chasing the threads with a die or thread file may be all that is needed.

If the threads are truly damaged, the damaged area can be filled with brass and new threads cut into the brass. This is a job for a framebuilder, but it's not particularly difficult if you have the tools and equipment. The threaded portion of the steer tub is far enough away from any visible painted areas on the fork, so paint damage is unlikely from the brazing heat.