Here's a picture of the rebuild kit made available through Beck-Arnley
The Beck-Arnley part number is 713107.
Use an air compressor to remove the pistons. Cylinder access is best if you separate the caliper halves, by removing the four bolts. Make sure the pistons are not pitted. New pistons are available from Walloth u. Nesch, GbR. Then lightly hone the cylinder walls, so the pistons will slide without binding (just enough to remove the rust). The O-ring between the caliper halves can be replaced with a Buna-N type available at most hardware stores. If you mess up the bolts when they are removed, use new M8 cap screws of the appropriate length. When re-assembling use some brake cylinder lube - makes life a lot easier.
This is a job that will save a lot of bucks and you won't have to worry whether the rebuilder did a good job.