I just checked Sirius, couldn't believe they didn't have carb kits for 77/78 Keihins.
Here they are
KH-0982N Show Picture Carburetor Repair Kit CB750F SUPER SPORT 1977 $ 22.00 IN STOCK
KH-0983N Show Picture Carburetor Repair Kit CB750F SUPER SPORT 1978 $ 22.00 IN STOCK
K and F's are the same for 77/78 only diff. is jetting.
But what parts are you looking for? Should only need at the most bowl gaskets, unless basket case.
John