Need pictures, what's it made of?
Why does the retaining wall need to be repaired?
If it’s over 4’ high, it’s Building Permit time.
If so, you’ll need to get a design professional involved to comply with the Codes & develop the specifications and permit drawings.
That requirement is per the US’ IRC Building Code.
Hydrostatic pressure is usually the cause of the failure.
At 120 years old, it’s time to replace it, not repair it.