There are no mods, to my knowledge, that restore the existing fort ruins. That would cause way too many conflicts. Especially since many forts have quests tied to them.
Instead, go to www.tesnexus.com and search for "fort". You'll get ten pages of mods, many of which add one or more working Imperial forts across Cyrodiil. I can't personally recommend any, but take a look for yourself!
And be careful when downloading and installing. Not all of them may be compatible.