This isn't a matter of starting an editor from scratch.
Source is available.
A port of GTKRadiant to OS X is well under way, if not really working yet. For more information, visit the mailing list . It's buggy, and the interface has some glaring flaws. But it's there. The author of this port has moved on to other things, leaving his work in the hands of a few people, like me, who are now trying to find interested developers to finish what he's started.