Basically the title, can you use a Cybrid steering rack with a regular engine-driven pump and fluid? Or do I need to replace the rack?

Yeah it just needs fluid pressure. It’s supposed to use cybrid fluid specifically made for an electronic pump driven rack but people have used others fluids for years now. I’m unsure what fluids folks are running though I’ve always used the original stuff. But I doubt it matters much except maybe to the seals. Exmark commercial mowers use synthetic 20w50 engine oil for their hydraulic pumps. Lol.