I think you wont get a 140/70 rear tyre any more. At least nothing like a modern sport tyre.
The Saturno rim is a bit small for a modern 160/60. For some old Metzeler it was ok. But rims are getting wider and wider over the years.
So 150/60 would be a good choice.
And you can get this easy.
@Pete Dragons??? They also have gone many years ago
What's on the catalog nowadays?
Diablo? Angel? Can't remember the name of the super-sport tyre (never need such a thing)
But nearly every major brand has something 110/70 and 150/60.
Pireli, Metzeler, Continental, Michelin, Bridgestone
ciao Joachim