Woburn police want the public’s help to find a Winthrop man they say assaulted a cop and hit an elderly man with his car while fleeing the scene of a larceny.Tyson Delgado, 29, was being questioned by police around 9 a.m. yesterday about a theft at a business near New Boston and Merrimac streets when he assaulted a Woburn cop and took off in a white Chevrolet sedan, police said.While trying to make his getaway, police said Delgado struck a 65-year-old man in a crosswalk on Main Street and continued on several hundred yards with the man on his hood before he fell off.