There are lots of similar looking slave cylinders out there. Just check that the new one has a bore of 13/16in (~20.1mm - 20.6mm).
Don't forget to pack the areas around the pistons (under the rubber gaiters) with red brake grease to prevent future corrosion.
While you're at it why not fit an easy-bleed nipple to the new cylinder. You need size M7x1.00mm and it will make bleeding the rear brake so much easier.
