Plenty of websites dealing with victorian and specilised covings and mouldings.
If you are considering taking a cast of original it needs to be done by experts.
The question asked about replacing all is something you should decide, original feature always nice to have if you like that sort of thing and normally adds value to property.
Check the internet do some phone calls, add up pros and cons, see what makes sense.
But you are not asking the imposible but it may cost plenty.
sorry can't be more helpful.
But best of luck
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