You might want to check out a couple of retail websites, to see if you can buy replacement elements. That's a filter than MSR no longer makes. The MSR website, under water filter accessories, indicates that the "Miniworks/Waterworks" element is available. However, as I vaguely recall, the filters weren't exactly identical - it might be worth a quick email to MSR's tech support to make sure there's no problem replacing the element in the Waterworks with the Miniworks element.

The Miniworks element costs $40; if you buy the filter, chances are pretty good that you'll replace the element it comes with sooner rather than later - at that point, you'll have $70 in it. Personally, unless your budget is very, very tight, I'd spend an additional $20 and buy a brand new Miniworks ($90 at REI.)