Toronto’s favorite black sheep, Mayor Rob Ford, is rising to the defense of fellow Canadian Justin Bieber, who was charged with assault Wednesday for allegedly hitting a Toronto limousine driver several times in the back of the head last month. The news broke a few hours after the teenage pop star’s attorney entered a separate not guilty plea in Florida to drunken-driving and other charges. Ford, who has admitted smoking crack while in a drunken stupor and is being sued for supposedly orchestrating the jailhouse beating of his sister’s ex-boyfriend, defended Bieber when a US radio host called Bieber ‘‘Canada’s worst export.’’ ‘‘Well, you know what, he’s a young guy,’’ Ford said Thursday. ‘‘At 19 years old, I wish I was as successful as he was.”