For all the money you'll spend on doing this conversion, you'd be much better off just selling the 18 and buying an F18, Tiger, or the like. If you want a bigger boat, then a Nacra 20 would fit the bill as well. Also, you'll have an actual fleet to race against with the F18's.
I have all the stuff for the conversion, including an F18 spinnaker, spin pole, and midpole snuffer (hand made), so I know what goes into it. We had it on a Nacra 5.5. It takes quite a bit of effort and money to do this conversion, and in the end just buying a boat built for a spin is the way to go!
Anyway, didn't you just sell the fox?