If site have a lot of fire doors that have been in for over 20 years and do not have a copy of their fire certification would the doors need to be replaced or could they just have them inspected and then maintained if required

Even a new fire door does not have to have third party certification, so as long as they are inspected and maintained by a competent person and replaced if genuinely beyond repair then that should suffice.

There are competent persons that have the credentials to inspect fire doors, even where no evidence of performance is held by the building operator.  The fire door inspector should provide a report detailing which doors are compliant and including a description of the work necessary to bring non-compliant doors to the required standard.