This action does not flout any TP regulations. What needs to be determined is the safety issue. If LTA has the balls, they can go ahead and challenge someone in court.
I am confident this swift is a very safe car. Engine modifications of course is not legal. But removal of car seats and installation of anti-roll bars surely only make the car safer and stronger. If LTA brings this issue to court and cannot come up with a good argument (ie. lack of safety), they will become laughing stock. Then it becomes a matter of they say so you obey. Like that also can meh?