As hamster said new screw on freewheels are widely available, but mostly cheap "for e bike" use ones. I mean they work, but on a hope Ti hub I´d think they would be a bit underwhelming.
Period correct ones in mtb gearing combos are harder to come by and do not come with HG cogs if you fancy such modern extravagance. Top of the wishlist would be Shimano Dura Ace 7400 or Sachs Maillard.
Normal cassettes (for cassette hubs) do not fit and they mostly come with manufacturer specific sprockets, so no cross compatibility either. Good thing then you normally can´t get them without sprockets.