My '76 F has a Motad 4-1, thier website lists exhausts for the whole SOHC range, but the page title does say January 2002, so I 'm unsure if they still do them.
In case any one is interested the part numbers:
CB750K0-K6 (Sohc) M7H
CB750F1 (Sohc) M7HF1
CB750F2 (Sohc) M7HF2
CB750K7 (Sohc) M7HK7S
CB750KZ M7HKZ