Some sources claim that both WKS and XLS formats were supported in previous versions of Microsoft Office (2007 and older) and so if you have access to them you can be successful with wks to xls conversion. Microsoft Excel 2010 and later cannot open or work with older Works spreadsheets (.wks), regardless if they come from Microsoft Works or old Lotus 1-2-3.
Open source programs from the LibreOffice and OpenOffice.org bundles still support WKS format and can be used to successfully convert spreadsheets from wks to xls. Same probably goes for Corel Quattro Pro that supposedly still have support for the old MS Works formats.