If the overall mission is to get set up to use cheeper/more easily available performance cats, or even if the type of thing you need just isn't available for the micra, then how about modding the system you have to suit, get a say 200sx cat from the scrappy with the flanges and a bit of pipe either side, cut the existing center section to suit, and weld/get welded in the flanges with the correct spacing for the "new" cat. This could always then be replaced with a higher flow version at a later stage, knowing that you now just need to buy the cat for the other car.
Fingers crossed that there is a straight bolt in mod though!