Hydro brakes can theoretically use any liquid – even water is noncompressible, it can just corrode components and boil, so it’s performance isn’t great. Use veggie oil instead and you’re fine. Grips look like they’re just string wrapped with tape or something. Saddles can be made of leather. Tyres can come from natural rubber trees, the way they were always made before.

Interestingly, bicycle mechanics in Cuba already effectively live in the post-oil age due to the embargo. The way that they fabricate and adapt the parts, tubes, tyres, etc… that they need to keep bikes rolling is truly a marvel of ingenuity.