Well, you know one thing now, don't trust the original owner's repair work. This means a lot more work for you to do in your restoration as you will have to check and question how things were properly "together" before you started. Hopefully this bit of bother with the rear MC is the extent of strangeness you find.
One of the great things about the internet is many of the dealers now have the BMW "microfiches" on line for free. While they are not a guide to how to put things together in themselves, they can give a good idea as to what is needed and a bit of the order things go together.
Also, there are a lot of new and used parts shops online. Some of the airhead stuff has become NLA but there is a lot that is coming back on the market by places such as Seibenrock or EME.
Good luck and have fun on your rebuild. I am currently waiting for my frame and some parts to come back from the powder coat shop. I am overhauling my RT. St.