The early fastback mudguard has a rare plate at the forward end, seldom seen. I fitted mine with a rubber mount onto the rear of the battery tray
That's what I did, using an undrilled stainless Triumph fender. Seen here beyond the Titanic's rolling chassis.The early Fastback rear fender is pretty narrow, running on memory.
Could you use an undrilled generic stainless rear fender, chopping the front until you get the rear stickout that you like? With stainless, you can bash it to go around the oil tank, if needed.
Then you can mount the taillight/license bracket wherever or however you want.
Stephen Hill