Replacing biomes?

I purchased a world online which was created using worldpainter to be a nether island, but to my annoyance found out that interspersed around the nether waste biomes are plains, badlands and other biomes. They never got back to me with an answer (or at all) when I asked so I need to fix this issue myself, any recommendations on how to approach this?

I tested the world on my server and in Singleplayer 1.20.1 so I know it's not the version that is the issue. The map was created for 1.19+ so it should have included the new nether biomes.
Was this page helpful?