I agree it's much harder to get proper composition when shooting someone coming at you at 100km/h, as when shooting still landscapes. But nevertheless even this is possible, and personally I think sport photos without proper composition are crap. Ok maybe I look on this a bit too much from my perspective (sport shooting is my job afterall), but even for hobby shooters, I think they need to think about composition too, not just catching right moment.
But then again, right moment and composition are quite related in sport photography. Normally right moment gives you lot of help with proper composition already. But main thing is knowing sport, how they move, what they do, and most importantly, what you can expect next second or next 1/1000th of a second. This way you can predict what will be at time you will actually press shutter. And this way, you can set your camera so, you don't really need to think about composition anymore when it comes to that short moment, when you will actually press shutter.
Main point with this is choosing proper af point. This is main thing, where most of people make mistakes. Sticking to center af point is perfect way to make horrible composition. If you can expect what athletes will do, you can set af point so, that you will have wanted composition, and this way, you really just press shutter at right moment, and composition is there already.
So my main tip is, to predict what will happen, to picture your wanted photo in your mind, set af point so, that you will have wanted photo, and when time comes, just press shutter. And lots and lots of training helps 