The only things still available from Honda, and that is the route you should go, are the rims and spokes. It's been a while. but I believe the rear wheel being standard K7--the front is anyway. I know of no parts list for the Japanese K7--that's how rare it is. Work with the K7/F2 lists from here:
http://www.cmsnl.com/honda-cb750four_model14344/None of the other parts, but the tank, is available from the N.O.S. Honda dealers worldwide. Most parts will become available in Europe. Being stationed in Germany with a domestic U.S. military address, I can help if you find things on EBay here. I have both side covers in the right color in mint condition, but I will sell them only with the mint tank combined. Just be patient and check EBay every so often. The aftermarket ones will have to go. If N.O.S. parts pop up, they cost a fortune. The last headlight bracket I saw sold for over $ 400 for one. This is the last--bare--tank worldwide I know off (other than the two I have) available from a specialized dealer, wrongly only listed under K8, for $ 1,600.00:
http://www.cmsnl.com/honda-cb750k-750-four-k-1978-usa_model7244/tank-assy-nh-1_17520405770zb/Again, the exhaust is available new on EBay as OEM part--although not from Honda, Sankei made them as they did for Honda back in the day. You will have a hard time finding the right bracket as it differs, but the regular one will work. Again, I have one but sell the entire exhaust with all parts and cannot rip it apart. These remanufactured ones begin to disappear, and prices will go up again. A few years ago, before they were remanufactured, you could sell a NOS set for the price I paid for my K7 new back in 77.
I have the Honda Shop Manual for the K7/F2 and can copy and scan individual pages if you need help with something.
The brake switch on the K7 could be one or the other, depending on year and specs (K7 does not mean 1977 in every country). Even with the switch built-in, you can use the other setup, but why would you? If you need specific parts, I am glad to help to locate them in Germany. It's probably the biggest market for NOS parts. If you need to go with less expensive parts, start collecting and restoring them piece by piece.
For reference, this is the set I am selling. It's the color you need with the red pinstripe--not the blue. You can get decals and have your tank painted. With patience, you might find side covers that are not broken on EBay. In the end, this route will not be any less expensive unless you have the necessary expertise yourself. But if you just want to restore it to have fun with it yourself, and you are not looking at it as an investment, less than perfect will do. In any case, have fun. It's an awesome bike.