The slave cylinder you are dealing with is the most common one. It is 44.5mm. The kits are non-existent. You can get cups similar to it at hydraulic supplies. They are rarely the right material. It needs to be buna. Same as the brake cylinders. The other problem is that the cups you can get, are not shaft seal type cups with the inner lip....but since they are generally slightly too tight, they seal well. They are generally a hair too thin as well. You will need to get whats called a backing rubber shim to put behind the cup to take up space. These will be a b*tch to fit, but can work. The best thing to start working on, is a bracket to use the vanagon or Audi 5000 slave cylinder. If you have an extra 412 slave cylinder, have the back macined out of it to make a hole. The Audi cylinder can then be fitted in until the clearance is correct and then use several set screws or a plate at the back as a stop. I am working on that next. It should be really simple. The long slave in the book is 68-70 models only. It is almost non-existent on this continent.
The large diameter slave was also not very well designed. The dust boot had nothing to do with sealing fluid into the sylinder. If it is wet with fluid and muck...the seal in your slave is shot. The crimped ring on the boot is to keep the rod centered while you install the slave. Without that boot, it is almost impossible to install the pushrod without dropping it into the bell housing.
You will also notice that the portion of the slave cylinder piston immediately upstream of the spring...but below the cup....is the same diameter as the rest of the piston. This does not allow fluid to properly flow to teh void in the cup. It makes for a poor pressure seal...and high wear on the cup...which is why it leaks so fast. I use a file to cut 5 evenly spaced grooves from the spring ares to the seal seating area. It allows much better fluid flow. It is also a good idea to source a strip of nylon...or better yet Delrin or teflon. Get some about .060" thick. Take your best piston to a machine shop and have it chucked into a lathe. Have a strip/groove turned around its circumference about 1/2" above the seal on the piston. Make it .058" deep. Cut a strip of the Delrin or teflon the width of the groove. Make the strip the exact ling of teh circumference of the groove. You are making a glide...or packing to put around the piston. to keep it from cocking in the bore...which is also a wear issue. Ray