Antique cars and old bridges have one thing in common— when they break down, parts are hard to come by. The Route 71 bridge over the Shark River between Avon and Belmar in Monmouth County has been stuck in the open position since early March, when a mechanical failure resulted in “significant damage” to the machinery that raises and lowers North Bridge span. “The bridge was successfully opened to marine traffic on March 12 and is secured in the open position,” said Stephen Schapiro, a state Department of Transportation spokesperson. “NJDOT is legally obligated to allow marine traffic to pass, …