So yeah, it's just the Champion and Board that are corrupted now, but it is slowly spreading through the ranks. And the cheating involves a steroid-like drug which increases a Pokemon's power.
So why does your protagonist care?
Basically, the short version of my earlier post breaks down my suggestions into the following questions:
1. How long ago was this?
1a. If time has passed between the time the protagonist gave up training (as "in his younger days" -- the phrase you used in your first post -- implies) and present day, then why is he only taking action now?
2. Why does he even care? Remember, the "justice for the sake of justice" very rarely happens in a single individual, and even then, there's usually more than that as a motive. Not to mention it's usually tried and true, and frankly, it's not very interesting if you leave it at that. Huge numbers of stories have protagonists that go out and right wrongs because they're the
hero. Very few of them have protagonists that go out and right wrongs because there's a specific reason that has nothing to do with the fact that they're the
hero. (Yes, stopping cheating for the sake of stopping cheating would fall under "because I'm the
hero" category here.)
3. Does your character have a
plan to go about bringing down the league?
3a. What kind of world does your character want to shape
after the league -- which you've already stated to have a massive influence on your world -- is brought down? Keep in mind that the league (as you've said) is essentially the government and therefore needs to be replaced. That is, there is no happy ending if you don't think about this part because the only possible outcome if you
don't is anarchy.
3b. Given that your character
is essentially inciting anarchy, what kind of reaction will he get? Will everyone just flock to him and support him because he's the
hero, or will there be resistance against him because
he's bringing down the government?
I apologize if any of these questions sound biting, but I really think you need to sit down and plan beyond the "the league is
cheating, and it's my job as the
hero to stop it" idea. Writing a story that stops at that has been done before, and frankly, those times left a number of plot holes that would have been interesting to see fixed. A believable hero
shouldn't just go out and do things because they're right unless they're not particularly bright or interesting heroes. Stories like that become predictable. However, if you sit down and think a bit more about the above questions, you'll be able to develop a character who has more of a motive and who has questionable methods (because this character will essentially be an anarchist).
Don't get me wrong. Yes, cheating is bad, and it's logical to react to that. But so far, all he would know is that the champion and the chairman are corrupt, not the entire system. And even then, overthrowing the both of them just for this seems like a little much. Maybe at most, the protagonist would want to expose this to the media, but outright revolution?
Also, it seems rather odd that the champion would just
leave that kind of thing laying around. I would assume that someone at that high of a rank would make more of an effort to conceal evidence that could be used against them unless the champion isn't the brightest bulb in the bunch.