on any older bike, it's always a good idea to run a ground wire through direct to the battery and assume the frame is no longer able to be used for this purpose.

email (option): cas.vanderwoude@gmail.com

I would say you can trust the chassis for electrical grounding,
Even on an old M20:

If the battery in connected to the body (To the "front" frame, not the rear/bottom frame)
And the connection is solid- Say by connecting to the saddle spring bottom end nut,
As the frame is brazed and riveted all the way to the steering head,
Then if you would take a ground from the front gas tank screws to the headlamp,
Then your ground is pretty solid.
The same goes for the rear light.
If the ground is taken from the frame, where it connects to the mudguard,
Then there is no need for running wires back and forth from the battery.
This is my view, anyway...

Cheers,
Noam.

I am earthing the battery onto the front seat mount all the paint has been cleaned off so it should be a good connection I would hope.

email (option): derrick.kay@sky.com

All the parts on my bike are powder-coated; I took no risk and had a black earth wire running from headlamp to rear light, and connected all critical parts to it; it's not much work, and prevents a lot of searching for those bad earth connections