Hard to see something that isn't there
This is a fairly unique post and top clamp although the basic design is used by many. No surprise that you can't find a replacement on a bike made by a tiny manufacturer (who was bought and sold and their manufacturing "plants" changed over the years) many years ago.
What I would do if I really wanted this bike to be useable is to make the parts. Easier to start the drawings if you can find an image showing them. The clamp likely consists of two basic parts. Both are near halves of a tube. The tube is split down it's length. Notches for the seat rails are filed/cut into the split faces. A through hole allowing a bolt to compress the two halves together , with the rails between, and some sort of shaped nut under the part on top of the post that you still have. Andy