Alright, I'm sorry for the other anon, but here's the real reason it isn't easy:
Communication over the link cable port is supposed to go simultaneous in both directions. This means that each device will exchange and process a byte at a time, and the game's code may adapt its course of action after each single byte. This is _very_ slow when performed over the internet, where it can take a long time for a single byte to make a full roundtrip over all of the nodes. Over the 3ds' wireless it's decently fast, all they had to do is hack the rom to wait a tiny bit longer for the transfer.
Now, I know Jap Crystal had online functionality. It worked by sending a series of bytes through a peripheral, which would then communicate with the internet. This used a lot of different code compared to the usual link cable communications (send a series of bytes and wait for a response instead of expecting an inmediate response) and only works properly in the japanese versions, due to it being scrapped in the localizations (any leftovers of the code hardly work). There's been indications this might've been planned for the jap versions but if so, it's been obviously scrapped.
TL;DR: You have to modify the game substantially for it to be able to communicate over the internet, since such communication can't be emulated. It just isn't very practical for an auxiliary of nintendo to hack so deeply into an age-old, sloppily coded game without source code.