I've been doing the same thing with an action that increases the speed, and just several of them throughout the game that set the maximum speed slightly higher. I'm pretty new though, so someone else might have a better solution or a more thorough explanation.
Hey! So, I've got a game with random scenes in it. There are a couple of ways in which I thought of addressing it, the first one being to have all the levels in a certain time range have the same speed up powerup in it, and then in the next time range they all have the next one, etc. If you want to have the same random levels throughout, you could have a really small level with just the speedup in it, and have a bunch of copies of it in the certain time range, to increase the probability that the level actually pops up. Not sure if that makes sense, but here's a screen shot. The levels at the top are the ones with the enemies in it, the ones below are the speedup levels.
