The 'inner tie rod' you refer to is the steering rack push rod that the curved outer track rod end (with ball joint) screws onto. Here's Rimmer's drawing (linked direct to site). You can se the threaded socket sticking out of the rack.
The standard fix for this is a replacement rack unit complete. Certainly there is no MGR part number for this item which is regarded as an integral part of the power steering rack unit. There are companies who refurbish steering racks and some offer the various bits. Whether actually replacing this rod in the 75/ZT system is a practical proposition I don't know. You might wonder what else is worn if the inner ball joint has failed (rather uncommon).
TC