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Now that i have got my 80 KZ650 E1 LTD running, i need to sort out the front brakes. The master cylinder was left open and in a mess. I have removed the whole system, which needs complete clean and check. I need the master cylinder rebuild kit, diaphragm, 2 lid screws and brake pads.Z Power have said the part number is 43020 - 1027 but they dont have it in stock. They suggest changing the master cylinder which i can do but would prefer to rebuild the original if i can. ​​​​​​​Also, the rev counter glass has been broken. Is there a replacement for this or do you have to change the whole unit.Any help appreciated 
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Hi. Just so you know, Wemoto does a line of brake equipment - their brand - called TRK Braking. They may be able to supply you with a rebuild kit comprising a brake piston and seals for the master cylinder, plus the brake pads separately. Don't know where you'd get the brake reservoir diaphragm, but find the part number and you'll surely be able to source one from somewhere. The Wemoto rebuild kits (the ones with the piston) are not the same as the originals. They are a completely different design incorporating an Archimedes screw - not as good as the originals. They seem to supply them based on the bore diameter, and they provide a spring too. They are not as good quality as the originals, and they don't come with instructions - work out the orientation yourself. Nevertheless, they may work. It may be worth getting in touch with them and getting more info. If not, then just try and find a used replacement master cylinder/reservoir. In your shoes, I would forego getting the rebuild kit and just look for a used Kawasaki original. 

Regarding the rev counter glass - this can be fixed but it's TRICKY. You have to dismantle the rev counter. Use a sharp knife to bend the enclosure without causing damage - there are Youtube videos on this. If you succeed, you will remove the internals as a unit and will have access to the surrounds. From there you will need to carefully measure the diameter of the original glass, and order a piece of perspex/glass from Ebay. When it comes, glue it in, and reassemble, and fold back the enclosure with care. This is not a job I like - a used replacement will be much easier.

Often times, when the master is all grungy, the bore is corroded and pitted, thus it's not rebuildable.  Good masters are quite cheap on eBay, though, so there is a simple path forward.  Get a master with a 5/8" bore (or 16mm,) if your bike has two front discs.  Or a 14mm master if the bike only has one disc.  I found a low mileage EX650 master for use on my 750.  

And while redoing the brakes, be sure to pop out the pistons and clean inside each caliper.  Lube the sliding pins with high temperature moly caliper grease.  And replace pistons if any are corroded and pitted.  

Lastly, replace the old brake lines, because they are sure to be all filled with crud.  

Good luck

Before buying anything, take it apart & inspect the m/c piston bore as suggested by Nessism.  Do not damage the plastic piston liner as that part is NLA >> the tabs are fragile >> press in one of the tabs from underneath via a rectangular hole & finagle the top tab.  Locate all of the parts beforehand.  As for the m/c piston kit 43020-1027, AFAIK it's not available & K&L doesn't offer a kit. However, a KZ650B kit might fit.

Possible alternate piston.  You might be able to use the kit from a 650-B2. It appears to be ver similar.
If the piston is too short or something, you might be able to swap the rubber bits onto your piston, assuming yours isn't corroded or shot. 
Pic is of the 650-B2 kit from Z1Ent. I guess it's still available, but last time I looked they were pricey. 

Hi Deon, I am also in the UK and have a unit from an 82 1000Ltd that I am not using in my build. Ley me know if you think you can use it