Hi.
I've been through this with my MK3.
Check the pics below.

The left pic is the old and new ones off my MK3. The original was a PBR P4977 but they no longer make them. The new mob making compatible ones is Protex, available through Repco. The part number is P5140.
The pic on the right is also a Protex part and is part number P7226. Both were around $35 from memory.
Both of these and the original PBR unit use the same rebuild kit, which is K655S. There's only a new piston seal and rubber boot in it, so if you need the actual piston, you may have to buy the complete unit. The actual circlip would be a standard item, suitable for a 7/8" bore.
Here is a link to the original post.
http://remlr.com/forum/index.php?topic=2816.msg28801#msg28801Greg.
Oh just remembered a tip that may help.
Bleeding this thing almost drove me mad. I finally found that the best method was to plug a bit of clear hose onto the slave bleeder, run it up and into the master cylinder reservoir. Crack the bleeder just enough to allow fluid through but only under a bit of force.
If you have a MK3, open the bonnet, reach down and start pulling the clutch pedal in and releasing it. If you have a MK4 or F1/F2, have someone sit in it and work the clutch while you monitor/top up the brake master.
This will pump the fluid around, so it squirts out the pipe and back into the reservoir. Wear protective glasses. It splatters a bit as the air comes through.
Stop now and then to top it up when it gets low. You will notice that the fluid looks fizzy. Once there doesn't seem to be any bubbles coming through, either have someone jump in and pump like mad for a bit or really give your arm a workout. You will see a point where the fluid in the pipe and the reservoir are totally clear, no bubbles and no sign of fizzyness. Your done.
If, when you pump the clutch, you see the fluid surging backwards and forwards in the pipe, tighten the bleeder slightly. I was able to fiddle around a bit and got to where the bleeder would allow it out, but not back.
The routing path of the pipe seems to trap pockets and bubbles of air. Doing it like I just described was the only way I could get the pedal hard. I tried the regular push, crack, close, release method, the reverse flush method and some weird ideas I came up with on the spot and the method above ended up working and the pedal is still hard now, almost 2 years later.