Honda-SOHC

My next question concerns interchangeability of parts between the 1975 400f four super sport and other Hondas, like the clutch for example, I need a friction plate and they are £50 or so for a set ! excuse me ! now I know that bits for these old bikes can be expensive but this is a bit strong, perhaps you good people can set me straight on this.
Many years ago I set up and ran a motorcycle breakers with a school friend " Bike Busters " at Wotton Rivers near Pewsey in Wiltshire, but that was quite some time ago now, anyway alot of stuff was cheaper then.
Cheers all.
Steve.

Hi peeps,
I am a lifelong biker and have just bought a crusty old 1975 cb400f super sport, my first step is to give it a good clean and then see if I can get a tune out of it with a view to riding it over the winter.
It does have an unusual locking petrol cap mechanism that needs some kind of hollow hex key to operate, David Silver was not able to shed any light on it, I was wondering if any of you guys knew about such a thing, it is a standard tank and all looks standard ?
Cheers all.
Steve.
username stevo.