On an engine that takes the early-style (pre-TS50000 "bomb shaped") starter, replacing the starter with one of the gear-reduction modern ones will allow greater room for installing a set of headers on the car, and will provide much better clearance for changing the starter after the headers are in place.
With the original starter, even if the header pipes clear the starter, having exhaust-temperature heat right near the end of the starter is not the sort of thing that's good for any electrical component.
The bomb-shaped starter also does have a bit of a weak point in the rubber coupling that's part of the drive: it can slip if the rubber bonded inside it breaks loose from the metal. I vividly remember pulling the TR3 starter on below-zero New Hampshire mornings to jam small pieces of wood in the starter drive coupling to get it to transmit enough power to get the car running. Too young, inexperienced, and impecunious to just get another rubber coupling back in the dark days before parts became more available for the early cars.