Most of the mario kart franchise gets looked at by hackers but of the vaguely mainstream stuff (so home consoles and handhelds) it seems the GBA effort is looked at least of all, and most of what I have seen is aimed more at speedruns. That said this might be more as while every other mario kart went on to enjoy various amounts of popularity then the GBA effort kind of stopped being talked about when either double dash or the DS effort dropped, to say nothing of the plethora of other fun kart games.
As far as custom items go I tend not to see that in any of them unless people are using it as a means to recreate other fun kart games (before people declared mario kart the greatest and borderline perfection, something which I am still confused about, there used to be a fairly burgeoning selection), or remove aspects from the game.
Harder AI is always a fun one in these. Sometimes you have simple variables or ability to tighten the rubber band but it can also become a silly hard hack (programming AI is seldom easy, programming in assembly is not usually regarded for its ease, hacking a game is not always easy and a combination of the lot does not help matters). That said I don't know what goes here -- it might well be fairly simple variables like we saw in some of the other hard mode hacks.
Custom tracks then. For the 3d efforts they tend to be broken up into the model itself which is the track and the data for it (see something like KCL files for an example, this being done as trying to get a game to manage 3d collisions is difficult and to try to figure out what is hazard, dirt, road, jumps, pickups and whatever else is worse still) and while functional equivalents do exist for the likes of 2d platformers I am not sure how this would have been worked.
