Attached is a file of a page from the Tomos A3 workshop manual. This shows a picture of the tools used to do it, and the person in the picture is starting to remove it. The sprocket holder can be made with two pieces of flat metal about 1 inch wide an about 10 inches long. Join the two pieces together at one end by drilling a hole through the ends at the same spot to put a bolt through and tighten together with a nut. Drill holes on the other two ends to put one bolt in each of the two end, and a nut to hold them in place.