if that doesn't solve the problem, then you can replace the freewheel with another. Sun Race, IRD, Shimano....6 speed recommended.
Unless the OP respaces his hub and redishes the wheel, he's limited to a 5 speed, or ultra 6 (6s with 7s spacing). Depending on the hub's current freewheel spacing, he might need to add 1-2mm on the right for a 6U freewheel, but not enough to redish the wheel.