I don't know about any mod that would suit your needs, sorry.
But I don't understand why you would fight the westerners on their terms. The Khwarez and Cuman units are not obsolete in the late-era. Their arrows might not be effective against western heavy cav/inf, but they will slaughter their pikes and missile & gunpowder units. Their light infantry do equally well against those units. Khwarez has a couple of strong heavy cavalry units who are a match for western heavy cavalry and they will wipe out their heavy infantry easily. Horse archers are excellent artillery killers.
Their actually doesn't change that much, tactically, for eastern factions. The stuff that leads to victory in the early period will continue to do so in the late period. Heavy armor isn't everything
