Motorcycle and Scooter appreciation

Just connect the earth to the body. The mounting screw for the new indicators should be sufficient.

I might go inside the headlamp and trap it under the headlamp mount nut (which acts as an earth for some stuff I think) since the indicators will be mounted to a powdercoated bracket most likely. Between the metal nut and the plastic headlamp shell would still earth it right?

or you could splice an earth wire inside the headlight bucket and connect them properly. bad earth is likely the most common cause of indicator failure so it might be worth it.

Having done this before, do not. The earth will be variable at best at it will be earthing through the headset bearings, Run a back wire from a good earthing point.

Where abouts are you in london?