I think it's a fair price for the car.
The front and rear seats aren't original, but can be replaced easier than if it were an '87 since there are many more Omni's for donor interiors.
The radio and speakers aren't stock, but you can still find the correct AM/FM Stereo Cassette and grilles.
There was a mention of the engine being rebuilt, but not replaced.
The addition of the boost gage may require replacing the A-pillar molding, but that's not a major thing.
The undercarriage looks extremely clean and the paint does too.
The NADA listing says $21k for that condition (yes I know some have said the NADA prices are inflated, but this one went for $3,650 less).