12" s are still available but for the availability and the cost you can get 13"s, it's not really worth it in chasing 12"s unless youreally want to keep it original.

and yes you can just put the 13" wheel and tyre straight on to your car, you'll have no issues or problems with tyre clearance. if you get some tyres with tread you may even find your 1/4 mile time will be better due to added traction and the bigger rolling diameter of the tyre