Having a new double splined shaft made would probably exceed the cost of another complete winch.
The part number for the shaft is Hyster #35572.
Long way from you but they may have the part...try Northwest Tractor Parts in Boring, Oregon.
Says they have two....
http://www.nwtractorparts.com/inventory.htm?http://www.machinerytrader.com/list/partssearch.aspx?pcid=883571&dlr=1