We reproduce, and repair stone ornament faithfully in its historical context. Ornament should never be completely symmetrical. This makes the ornament boring to the eye.
It's more aesthetically pleasing to aim for balance, rather than symmetry.
The ornament to be repaired or reproduced, is first modelled in clay, or wax, (usually on site) then carved in a suitable stone to match the original material.
Even small areas of ornament can be repaired using modern resin and stainless steel dowels, as long as there's sound stone to attach a repair.
There's often very little left of ornament in rottenstone. So experience, and knowledge of art history is essential to reproduce any ornament.
Personally, I believe the survival of the craft, is as important as the ornament itself.