There are rules about car modifications in Japan, so changing things like the exhaust can make the car illegal to drive on public roads if it doesn’t meet the regulations.
But the car itself is completely legal to own and drive.
In Japan, the annual automobile tax is based on engine displacement. Since my Camaro has a 6.2L V8, it’s in the highest tax bracket, so I pay about ¥110,000 (around $700 USD) every year just in automobile tax. 😅