Cocotron[1] attempts to be up to part with Cocoa though. I'm not sure if it would cover everything needed by TM, but it would be a pretty decent start.
Considering that the only reason GNUStep isn't considered a totally failed project is the existence of Hurd, I wouldn't bet my money on creating a reasonably up-to-date cross-platform Cocoa/OpenStep clone. Try to see if the base Objective-C++ is working alright and create a GUI-independent layer, at least that way you'll get your native look-and-feel for free. But looking at the code, I think that'll excisce a lot of the core TM tech, so whether it's worth the conversion effort instead of just starting brand new is a pretty good question.