What type of legal engine modifications can be done to your engine to increase speed and performance?

Well, legality depends on the state. Idaho for example doesn't give two shites about what you do to your car, I had a ridiculously modded civic as well as a jetta that did 155 and the cops never stopped me. Except once, and they say it was a cool car and let me off. It all depends on the state. Look up your local regulations.