This adventure, the gate of firestorm peak, has "weak " trolls in it. An easy way to convert these to 5e is to give them the stats of a normal 2e troll... which would be quite weak compared to the 5e one!
I’ve actually never converted a deepspawn, but if I did I would use a roper as a base, make it amphibious, then create/adapt legendary & lair actions from the AD&D version. Spawn monster. Hold person. Throw treasure hoard items. That kind of thing. Mechanically it’s a straightforward enough critter but the real key is it’s story and encounter presentation. Foreshadowing that monsters the PCs face aren’t quite right...are touched by the Far Realm...obey some malign intelligence...would be good. And making the encounter in a partially aquatic environment could make it scarier.
I just found a DM's guild product that converted a lot of the monsters in that adventure for 2$. Money well spent IMO... I'm not 100% happy with all the conversions, but it's easier to tweak them than do a full blown conversion IMO.