My understanding is that the F18s are now running kites with a luff length of about 340". That may well be smaller than the kite that came with the boat, but you should probably measure the existing one. The foot length of a modern kite will probably be shorter too. So I think an F18 kite would work fine for you.

If/when it comes to getting a new kite you should talk to a good beachcat-orientend sail maker about making a custom kite for your boat. You may find that a custom designed sail does not cost much more than an "off-the-shelf" sail.

I've ordered a SNU snuffer for my boat, should be here on Monday. I've heard nothing but good things about it, but don't have personal experiance yet.