TOPIC:

How hard would it be to retro fit this www.z1enterprises.com/yamaha-xs650-maste...0-D2%20650%20SR/1979 on to my 1979 KZ 650 SR and what else would I need to do it?  The stock rear master cylinder is leaking and I have not had any luck finding a rebuild kit.   

The rear master on a SR would be 14mm, that one you pointed out is too small. It also has that weird pivot attachment, you would not need that. A used modern master would be a better choice and fit easier.

I tried a rear master from a 2007 ZX6 but at 1/2" (12.7mm) it was too small for the caliper.

I ended up using a 2005-2008 Vulcan 1600 rear master which is 14mm. Both fit the stock mount bracket.

You may have to fiddle with a mount for the fluid cup and the brake actuator rod, but the master works like stock.
If you stick with Kawasaki you can get replacement parts.

If the SR has the same rear caliper as the C3 then you can use a front master kit for the seals.

www.z1enterprises.com/master-cylinder-kit-z1.html

Z1 part # - 200-2015

I just did this on a 1979 KZ650 C3 and it worked well.  Assuming you are looking for cup and piston seals.

The other option is to measure the cup seal and find a similar size from a different bike.  There are a couple threads on here about the same issue.

Corrosion in the master bore renders the core junk, regardless if you can find a good rebuild kit.  I've seen more junk masters than good ones when purchasing 40 year old brake system parts.  I'd go straight to something newer, and then work through the conversion, unless you come across a cream puff core, and those are getting RARE these days.