I'm not sure how bulky it would end up being, and I haven't messed around with connecting two of them directly (rather than through a router) so idk how low the latency could be, but you could use esp modules to do this I'd imagine. Something like the esp-32 or esp-8266, or the smaller board on them that actually does the wifi.
I may have the names a little mixed up, but that didn't seem to matter when I went and bought some lol. In the attached picture, the smaller board you see you can buy, or you can buy it attached to the bigger board in the picture. The smaller one is what actually does the wireless communication, but the bigger one is easier to work with since it has a usb port. The youtubers I watch that talk about these boards sometimes are Andreas Spiess, GreatScott! and bitluni. They're basically wifi arduino boards if you know what arduino is. The esp-32 is newer than the esp-8266, but an even newer module could have released and I not know about it yet.
You can get them real cheap from aliexpress, with kinda long to long shipping time, or spend more on them from another company but I'm guessing get them quicker.
Now if you wanted to make the gba talk to an actual wavebird receiver, you'd probably have to decode the wavebird's communication and if the link cable port doesn't just automatically spew out data about what buttons on the gba are being pushed, you'd probably have to mod the gba physically to get what buttons are being pushed out of the system. It sounds like you just want a wireless adapter for plugging the gba into a gc/wii, which should be doable without modding the gba, you'd hafta learn how to program these wifi modules and figure out how you should attach the link cable ends to them. But shoot, you could probably just mod the connector end of one of those already existing wireless cables to fit your console if that's all you want.
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