If I replaced every door that had cracks, I'd replace every door. A new door is rarely going to replicate an old one and most old ones are built from a much better quality of wood.

I don't know how to distress a door, but you could always look at architectural salvage places to get old doors.

a wood door moves as it gets dry in the winter and then absorbs moisture in the summer. If it is painted ( unless there are lots and lots of layers), there will cracks.
New doors will not have the patina of old ones, even if they are custom made to match.
There are excellent people with websites on the internet, and actual stores, who know lots and lots about old door knobs and latches. They can probably advise you, as well as sell you parts to fix what you have.

we had all our trim antiqued by a fellow that knows his job well. He painted with primer,painted with paint he then sanded areas that get most wear & tear,That brought up primer some,he then put clear on,and antiqued The mix he made up for the antiquing..Beautiful.You cant tell which is new which is old.we had mouldings etc that needed replaced.