I HIGHLY doubt this would be an easy port. But it would be freaking awesome :P
http://www.openttd.org/en/The original windows game has all the files you need to play OpenTTD
http://www.abandonia.com/en/games/240/Transport+Tycoon+Deluxe.html
I've locked some on the libraries that Open Transport Tycoon depend on and I think that hardware related audio/graphics libraries requires alot of work before we have a working NeXTSTEP port. I also think that a newer compiler will be required in any case, so I think that we're out of luck...
It's way over my head anyways...
d'oh. Oh well, thanks for looking at it! :)