A displacer beast can't undergo true ceremorphosis
As indicated in sources from 2e's The Illithiad to 5e's Volo's Guide to Monsters, ceremorphosis only works on humanoids, and then only on select kinds. A displacer beast implanted with a tadpole could become some sort of aberrant monstrosity, but not a true mind flayer, and would not be accepted as an equal in society as a result, any more than similar creatures such as the mindwitness (an implanted beholder) or urophion (an implanted roper).
...but it can become something pretty awful
The 3rd Edition Fiend Folio offers us a half-illithid template to reference, which has some solid basics as well as the aforementioned face tentacles. Easy way to go, really, but if you want to play with the concept a bit more, here are a few suggestions:
• Dragon 281 offers a table of sample illithid names. From this you might want to call your monstrosity a lugrithion (roughly "feared darkness"), maanzehoon (roughly "the one outside law"), or xallibosk (roughly "master of pain") if you'd like to match the general cadence and "ring" of illithid critters. I'll be using lugrithion in the below.
• Rather than the implantation transforming the face to be capable of cranial ingestion, it should transform the displacer beast's own tentacles; perhaps it grows a second pair, and each one's pads become lampreylike mouths that can be used to pin down a potential victim for extraction. I'd suggest maintaining a hold with two of these might be enough, since they're larger surfaces; this immediately makes the lugrithion a much more melee-cadenced foe, as they know they are considerably more deadly and have the spare tentacles to threaten a second foe at the same time. In exchange for this increased melee deadliness, I would not carry over the mind flayer's stunning effect on its tentacles.
• Adjust the spell-like abilities; remove dominate monster and replace with something fear-based as a way to emphasize the calculating predation of the lugrithion - this is a combination of the cold and unfeeling mind of an illithid with the malice and catlike attitude of the displacer beast. You could also choose instead to emphasize its illusory capabilities.
• Retain the magic resistance, telepathy, and mind blast.
• Physically, the creature would retain the dark coloration of the displacer beast but would be hairless and slightly slimy, with a foul sheen glistening from it. Its eyes would be glowing white and in place of its muzzle would be moist smooth flesh that throbs and flutters as the lugrithion breathes, since it has no need to vocalize and its food is ingested through its tentacles. Its digits might also be a bit more elongated and webbed, spreading in a manner disturbingly reminiscent of a clammy human hand.
• In short, a large bestial thing that's a mix of a panther, a Xenomorph, and Gollum, with lashing tentacles on its shoulders that can be used to extract brains.
• The attitude of the lugrithion, as noted above, would combine the cold disconnectedness of illithids with the sadistic impulses of the displacer beast, producing a nasty, extremely intelligent predator that uses stealth, confusion, fear, and disruption long in advance of making a direct attack, purely to "season" the brains of its prey with fear. It would bear no true loyalty to any other being and would remain in the community out of either fear or convenience. Between telepathic whispers, manipulation via illusions or fear effects, and the awesome power of a mind blast, a lugrithion should be an exercise in anticipation and dread.
Why don't flayers make more of these elsewhere? Well, perhaps they do, but how wise is it to upgrade a powerful apex predator to make it more psionically adept and physically vicious while also leaving it with absolutely zero loyalty to you? Enough lugrithions have likely made the calculated decision to snack on their own creators that they remain an uncommon encounter even for dedicated delvers of the Underdark - and thus a perfect surprise for unwary adventurers.
Hope this helps!