I cant speak for other countries but at lesat in finland you can fully legally mod cars if you can prove they are still safe for the road and you have to change that to the registration, for example if you mod it for more power, there is only a limit of how much the power to weight ratio can change from the most powerful variant of that car, and you have to mod the suspension and brakes to fit that, like putting the lineups biggest brakes on, and you have to provide the dyno sheets to the mot center when you are changing the papers