You can buy a flange repair kit that can be installed onto your existing flange and will provide the means to attach your toilet. Here's a link that'll show you one. They should sell them locally at either Home Depot or a plumbing supply house.
http://www.quickfixplumbingproducts.com/spanflg.htm
Your only other alternative is to replace the flange, which would mean cutting a hole in the floor around the old one to try and replace it.