Hi MD Moz 1277,
Anglesea race is good fun.
Wish it was closer to home, just love kicking Hobie Butt.
I don't know for sure what issues you would have fitting a Nacra 4.5 Spin kit, I don't recall having ever seen one. But the things too look out for would be weight of the parts, length of the pole and dimensions of the spinnaker.
A lot of cats are rigged with a longer pole than the Mossie uses which is 3m. Also if the spinnaker dimensions where larger it would make fitting it on the Mossie more difficult. If the foot is longer or the clew higher cut you couldn't sheet of the sidestay chain plate, which would be a pain as anything further back is going to get in the way of moving in and out on trapeze. Also if luff is longer this would require putting halyard point higher, which could test the strength of the mast.
Is it realy cheap or something? Otherwise I would buy one that you know will fit, I bought a Irwin recently just fits and works straight away, if you don't want to do your own fitting, Goodall will supply a complete kit.