A multitude of Suntour RDs should do the trick, big question would be long cage or short cage, dependent on size of your Big gear on the freewheel. Either available in the Parts Trading area (top of the page) or Parts for Sale (top of the page) or a local bike shop that deals in used bits, or.....EBAY.....

Welcome to the forum. PICTURES of said bike are ALWAYS welcome (IF not DEMANDED! ) ;-)