Starter solenoid

... fit then all (most) solenoids are similar. Some have an extra terminal on the switched side for vehicles fitted with ballast resistors and you don't need that. I used one of these:

https://www.almsolutions.co.uk/index.php?main_page=product_info&cPath=1_45&products_id=1879

Just about all starter solenoids are the same, just a very heavy current relay.
But an anomaly will show up when you try to make any solenoid work on the Electra. All solenoids I have seen need a live supply to operate them from the ignition switch. On the Electra this was supplied from the original Wipac Triconsul handlebar switch. No Longer Available. The repro Triconsul even though it looks the same does NOT switch live out to the Solenoid. SO you will have to arrange a relay via the ignition to make this switch work, details on aoservices.co.uk

Hello Rik,
I once converted an aftermarket Tricon switch into an Triconsul
 for my 1970 Norton Commando.
https://www.accessnorton.com/NortonCommando/wipac-tricon-to-triconsul-2017.22585/

I also have Electra barrels lying around if you are interested.
Regards

Uli
Ravensburg, Germany