First off, get MWEE (
Main Package,
Manual) and TESPCD (
Main Package) -- the former will handle savegames much better than the CS, and will let you deal with the raw hex of a plugin, and the latter will get potential problems sorted out when you're instaling a new mod (not in the "push a button, get back a clean mod" sense of TESTool, but through viewing conflicts and bad data yourself, then choosing which records to remove).
After you have the necessary tools, make a copy of the plugin and, using MWEE, remove all CELL records from it; afterwards, implant your choice of the DIAL entries for the missing topics so as to overwrite the ones currently in the plugin -- the obvious choice would be to use the entries from Morrowind.esm, but if you have a plugin that contains the relevant records (to the exception of the unedited instance of the plugin that started this mess), that would probably be a better source. Load up the game with both the original and edited plugin loaded, and, after a few AddTopic commands issued to the console, you should be set.