You can still get tubulars. They must be glued to the rim. No, not compatible with anything else.

If you wanted to run clinchers, like most bikes, the best thing to do would be to get some other wheels, and hang these up, or trade them. Probably hang them up, since as long as you have them, the bike is "original." As for getting more, you have fortunately walked into a vibrant vintage-bike community spanning four or five forums that loves this stuff, and French bikes of this era are not rare. So if you walk over to the "wanted / for trade" thread you will surely find someone.

Most clincher wheels of this era are "27 inch." and a wheel set should be easy to find. There are not many premium tires still offered this size. The fast tires we like are called "Pasela" and are fairly lightweight but only barely armored. Plenty of "keep it running" grade tires, though.

Changing it to the modern tire size, which is slightly smaller, is possible but more of a project