Looks like we should base it on the Pseudodragon, there are many similarities in its stats.
Its size (including 3' body length), 6" land move, AC, and see invisible ability are identical to the AD&D Pseudodragon, so we may as well just copy the SRD Pseudodragon's equivalent in those areas. (Tiny Dragon, AC 18, Speed 15 ft, Blindsense 60 ft., darkvision 60 ft., low-light vision)
Now for the changes:
Increase Hit Dice by to 3d12, with advancement 4-5HD (Tiny). So it's 3-5 HD to a Pseudo's 2-4 HD.
Its Magic Resistance is 30% to the Pseudo's 30%, so Spell Resistance 18 instead of the 3E Pseudo's 19?
Plus the normal true dragon's immunity to sleep and paralysis, of course.
Its phase-shifting ability is its big power. Since it says it works like a phase spider's, we may as well copy-and-paste the SRD phase spider's Ethereal Jaunt power.
The 1st level magic-user spells just look like it gains sorcerer levels somehow, but not all pseudodragons can do this. Maybe add a subentry for "arcane phase dragons" that casts spells as a sorcerer of a level equal to its Hit Dice?
Now, what shall we do about ability scores. A pseudodragon is:
Pseudodragon: Str 6, Dex 15, Con 13, Int 10, Wis 12, Cha 10.
I feel its stats should be a bit better than a pseudodragon, it has 3HD to the pseudo's 2HD and an intelligence of average-very (8-12) to a pseudo's average (8-10). Also, it will need a 12 in its spellcasting stat (Charisma if we go sorcerer) to be able to cast 2nd level spells.
So, how about the following:
Phase Dragon: Str 8, Dex 14, Con 13, Int 11, Wis 12, Cha 13
It has bite 1d4 instead of a sting.
Hmm, I'm tempted to increase its bite to 1d6 so its average damage (1d6-1) is almost the same as the original's 1d4 sans penalties.
The phase dragon doesn't have telepathy like a pseudodragon.
And finally, it has a confusion breath weapon instead of a coma-causing poison. That looks like it translates to a 10-foot cone with effects as a confusion spell (caster level = HD or HD+3?).