Actually, the original text says Str is 18(50), which translates to Str 19. I could even see some justification for a slight bump from that, depending on how things look later. In any case, aside from the Str, that looks pretty good.
That depends how you translate it. An AD&D Strength 18/50 is the next Strength category in line after 18 (arguing for a 19), but AD&D ability scores have a much wider plateau in the middle, so most stats don't get bonuses until they reach 15 (instead of 12 like in 3E). Using the damage bonus of +3 for that Strength instead of the raw number you get an equivalent 3E Strength of 16-17, which is what I went for, since it was closest to what you get scaling down an Ogre or Athach's Strength to Medium.
Still, I have no objection to boosting it to 19, we can always say Fachans are at the burly (300-400 pound?) end of Medium.
There are a couple of other stats that look east converts. Speed 9" => 20 feet, Alignment is probably Usually Chaotic Evil and treasure is pretty good (U or Z), so should be at least standard. (It's actually a lot for a single monster, so double standard may be more suited - but that seems rather a lot for such a monster).
So we're now.
Lesser Fachan
Medium Aberration
Hit Dice 5d8+5 (32hp)
Speed 20 ft.
AC 15 (+2 Dex, +3 natural)
Str 19, Dex 15, Con 12, Int 7, Wis 12, Cha 6
Alignment: Usually chaotic evil
Organization: Solitary
Treasure: Standard