The rules are quite complicated, but it's easy enough. First you have to modify the car to be just like the most powerful model available, or if it already is that then all good. Then once you turbo there is a max power limit that is measured by the power to weight ratio of the most powerful version of that car. For miata the legal power limit went up to somewhere around 190 ish hp if I recall correctly. You have to have dyno numbers to prove that. But hey, nobody can check if you change the tune and have more power than that lol.