For the 1938 racing season, Alfa Romeo created what many consider the finest pre-World War II sports car, and one of the most monumental automobiles, ever conceived.

Designed specifically for the fearsome Mille Miglia and grueling 24 Hours of Le Mans, Alfa Romeo’s new racecar employed a highly tuned example of the marque’s inline eight-cylinder engine with double overhead valves, double overhead cams, twin superchargers, independent four-wheel suspension, transaxle, and suspension dampers controllable from the cabin.

Alfa Romeo built four racing spiders for the 1938 Mille Miglia, all of which survive and are among the most hallowed of all vintage automobiles.

Every one of them belongs to a top collection and would no doubt command seven-, perhaps eight-figure prices should they come up for sale.

British classic-car enthusiast Stephen Muir had long been obsessed with the vaunted 2900B and, lacking the opportunity to buy one, set out to build an exact recreation under the auspices of...