I know of no official conversion, as of writing. However, were I to write a conversion, I would begin by comparing the stats from D&D 3.5, where the astral dragon appeared in Dragon magazine issue 344. I would then use that to adapt an existing 5e dragon (which for copyright reasons I won't replicate here).
The astral dragon is approximately as strong as a brass dragon of similar age, and we can use the 5e statistics for that as a basis, modified as follows:
- Breath Weapons (Recharge 5-6).
- Scouring Dust. A cone of scouring dust (as the standard dragon's breath weapon, except that the damage type is slashing).
- Psychic Wind. A 5 foot wide line of dark cloud. Affected creatures must make a saving throw or be affected by psychic wind, as per the Dungeon Master's Guide p. 48.
- Severing Bite. On a critical hit with its bite attack, the dragon instantly severs an astral traveler's silver cord, killing them instantly.
- Detect Extraplanar. The astral dragon has advantage on Perception checks made to detect creatures who are not native to the astral plane.
- Lair Actions. An astral dragon makes its lair on some chunk of matter floating in the astral plane, often the massive petrified body of a dead god. On initiative count 20, the dragon causes one of the following effects:
- All gravity within 60 feet of the dragon is nullified. All creatures' standard walk speeds are reduced to 10 feet, and creatures in the air who lack a fly speed simply float in place.
- Astral wind buffets all creatures within 20 feet of the dragon, pushing all creatures 20 feet in a direction chosen by the dragon.
- Regional Effects. The region containing a legendary astral dragon's lair is warped by the dragon's magic, which creates one or more of the following effects:
- Psychic Wind. Creatures approaching within six miles of the dragon's lair are affected by psychic wind (DMG p. 48).
- Tidy Nest. If a creature not native to the astral plane is slain within 1 mile of the dragon's lair, its body is banished back to its home plane. There is a 20% chance it will be sent to another plane. Any gold or magic items the creature was carrying are not banished.
Astral dragons are always of Neutral alignment. They consider themselves guardians of the Astral Plane, and consider all non-native visitors to that plane equally their enemies.