It's... Not that simple.
First of all, you'd have to find a mod that does that for 1.8, which I highly doubt exists. Then, even if you can, everyone that wants to play the map has to be capable of using mods(which immediately excludes a lot of people just based on their computers' specs). And after all that, you're now placing the playability of the map in more modern versions on whether or not that mod is willing to update.
Trust me, if I could easily use mods without the inherent troubles, I would've added custom NPCs a long time ago.
I can't imagine that they would be able to feed that whole city on imports given how isolated it is. Additionally, Incan cities were known to have terraced farms, which is why I chose to use that style of agriculture to start with.
Wait... What? If I place lava underwater, it will just turn into obsidian, not to mention that the concept doesn't really make much sense to start with. Why would they hydrate their crops with lava? Wouldn't they just harvest seaweed and/or other underwater crops? Or perhaps do their agriculture above water?