SHK Fund Management, a unit of Sun Hung Kai in Hong Kong, is marketing a US mortgage-backed securities fund for the second time. It first introduced the fund last summer as the first (and so far only) product under its alternative investment division, raising $220 million. But the fund's returns failed to meet its targets. Now SHK has brought out the fund's New York-based manager, both to explain and to raise new funds, because this strategy is designed to take advantage of the current difficulties in the US economy.