Russians still develop Energia rockets and engines. It is modular rocket system which included soviet space shuttle and some heavy orbital weapons. It has more powerful engines than F-1 used in Saturn.
It is modular system and heaviest configuration were close to Saturn, but are currently abandoned. The lighter configurations are in active use and share the same engines and other components. Plus they develop new types.
I guess it would just take a few months/years to develop Saturn replacement if there is demand.
RD-170 family is alive and well though. You'd still need a good sized hydrogen engine - a kind of SSME or RD-0120.
But may be you just don't need too big of a rocket. 25 tons on LEO could be enough for everything, and you'll launch that often enough so a single launch won't be expensive. You can assemble stations, create orbital fuel depots, even make on-orbit manufacturing if you want something really big in one piece.
It is modular system and heaviest configuration were close to Saturn, but are currently abandoned. The lighter configurations are in active use and share the same engines and other components. Plus they develop new types.
I guess it would just take a few months/years to develop Saturn replacement if there is demand.
https://en.wikipedia.org/wiki/RD-170
https://en.wikipedia.org/wiki/Energia