WorldGen Block Replacer is a mod that replaces blocks in your world with other blocks, which you can choose in the config. This happens when new chunks of the world are generated, and also works on preexisting worlds. By default, it does nothing. It also works with blocks added by mods.
I suggest that you do not replace any block with a block with a tile entity(furnace, piston, etc). If you do, your world will most likely become a lot slower. I am not responsible for broken worlds.
This mod can be run on a server without being installed on the client.
For details about WorldGen Block Replacer 2.2.0, see this post.