I have a fork with a 28.6mm steerer tube. It appears to be completely straight, with no flare at the bottom. I bought a bike with a damaged crown race. I bought a Jessica head set, but the crown race appears to expect a 30mm flare but my tube is 28.6 all the way. Can anyone suggest a solution?
Problems with the fork's crown race seat being under spec such that the race slides on and off without any interference are fairly common. Usually the gap is more on the order of 0-0.2mm or so. In almost all cases, this can be addressed by either knurling the seat with the Stein knurling tool, applying a retaining compound, or both. But I doubt either of these would work if the crown race area actually measures 28.6 as you say.
If it really is 28.6 all the way down, that implies a defective fork. It could potentially be addressed by getting a crown race seat from a framebuilding supplier and either brazing or bonding it on. The crown race seat being left off at the factory seems like the most likely explanation. Finishing it with either a crown race mill or an enormous, rigid lathe would be required.
You could also try a build up and knock down type approach with brass if it's a steel fork or a metal-reinforced putty otherwise. Loads in this area are considerable so there's some question whether it would work long term without breaking or crumbling.
A simple crown race that goes on and fixes everything doesn't exist, as 28.6 crown race seats aren't a standard.