Repair or replace a fire door?

Usually, repair. The exception is a door that was never a proper fire door to begin with.

Take the usual inspection list: closer's weak, seals painted, a couple of hinge screws missing, 5mm gap down the lock side. Every one of those is fixable with the door still on its hinges, and it's a short job, not a new door and a skip.

Now take a flat front door that's got two raised panels on it. That one can't be repaired into a fire door. Panels are thin spots, and the moment a door has them, the fire rating is gone. Spend the money on a plain certified doorset instead.

Same story if somebody's cut a hole in a door for a letterbox or a spyhole and not fitted a proper fire-rated kit. And the same if the frame has split along the hinge side, because there's nothing left for screws to hold.

If you can't tell which kind you've got, send a photo of the whole door and one of the problem. We'll say which way it goes.