Is it legal to remove parts before repossession?

Yes and no. If you have replaced the factory radio with a super sound system, you can put the original back. Same for fancy wheels or high-performance tires, etc, that you added after the car was financed. Remember, you are still responsible for any balance remaining after the bank sells the car. The bank is going to sell the car as fast as they can, which means they get less than it is really worth and any missing or damaged parts will make it worth even less.