searching for a datapack/mod - Multiverse-core

Hey,
I'm trying to migrate my server (latest version 1.21) from Spigot to Fabric. I've found a mod version for almost all plugins – except for "Multiverse-Core" (we use it to have a pre-built Superflat world).
I found a similar mod called "Multiworld", but it doesn't have a Superflat generator and doesn't allow me to upload my old pre-built world.
So I wanted to ask if you have a solution to this problem or if you know of an alternative mod or a suitable datapack.
Thanks in advance for any help.