I don't quite understand what is going on here. How are you going to remake TFC on a different Engine? That alone means it isn't going to be TFC. You mean Fortress Forever 2.
My advice is tweak TFC. Sort the Pyro, bugs, make some new maps (or edit current maps), models, textures and whatever else on this custom GldSrc engine (or some other modded engine I don't know about) Custom match modes/options built in will be brilliant, like nade radius etc :
http://www.moddb.com/engines/xash3d-engine
But Valve wouldn't allow this, right? Since TFC isn't free.