Sales have plunged at Spuck Bennett’s Harley dealerships. “We’re just trying to survive,” he says.   Like many cash-tight companies, Harley, based in Milwaukee, is finding that borrowing is difficult — and expensive. In early February, Harley announced that Berkshire Hathaway, Warren E. Buffett’s company, would buy $300 million of its unsecured debt. (Harley reported total debt of $3.9 billion last year, more than double what it held in 2007.) In exchange for his good na...