In NZ the washers are not available.
Blacks Fasteners here sold me two items which do the job:
WCSS12 M12 304 Stainless Conical Belleville Washer DIN 6796
WHA12264Z M12x26x4 Hardened Flat Washer ZP F436M (38-45 HRC)
Held the Belleville washer in some vice grips and slightly enlarged it with a cone drill to allow it to clear the threads. The hardened washer clears the keys on the motor shaft.
My original washer was cracked so I could never tighten it.
I removed the spark plug and filled the cylinder with a light cargo strap to lock the engine to help tighten assembly.
Hope this may help someone.
