Unless you have a really good model where your app's logic is separated from your GUI and it's not using platform specific APIs, you're probably just going to learn Cocoa and write it from scratch.
I agree with the above poster. It's better to start all over than to get done and realize no one likes it or will even use it because it looks like it belongs on Windows Mobile. The response the application TripLog shows that people won't accept that and are incredibly harsh about it.