Generally the advice on the care and conservation of Victorian and Edwardian ironmongery is the same as for Georgian ironmongery. If replacing missing ironmongery, look for a pattern in similar properties to copy.
Renewed interest in period features has led to an upsurge in the availability of traditional ironmongery patterns using traditional casting moulds and techniques. The internet has a vast choice. When buying from the internet ensure that the piece is solid brass, and un-laquered.
Ensure that the diameter of the knob is correct, i.e. important rooms have bigger knobs. The thickness of the rose is also important and should be as thin as possible at the outer visible edge.
All ironmongery should be of the same material, eg all brass or all black iron.
Resist the temptation to add extra ironmongery to a door unless necessary.