Be careful, you may end up devaluing your vehicle by replacing parts. A lot of body parts have stickers from the factory with the vehicles VIN. People who know Miatas will compare the stickers to ensure that the vehicle hasn't been in an accident. If you replace parts that should have the sticker, people may assume that the car has been in an accident.
Of course, if you are going to drive it until it is worthless, it doesn't matter.