Just get the 21 and it should be fine with the 18 CV-RX , then you will be able to see if you need a bigger bell or not and afterward's you can get the 23 bell that should be direct drop in .I don't know of any smaller spur's for the GT2 .
OS makes a 18 rear exhaust that can fit many cars but does not fit the GT2 except if you are ready to get rid of the pull start and get a starter box ,i think that for your car the easiest will be the 18CV-RX that will cost you less as you will not need a new manifold and a starter box but still will get you very nice powerand you will be able to pull wheelies on demand .