There are lots of fan pages around – some even sporting the out-of-print rulebooks in PDF form – that don´t get blasted by the current copyright holders. It always depends on the company´s attitude, though. Most times, they won´t give you trouble for a fan project unless you´re making money off it. After all, you´re advertising their product.
The only trouble you could expect would be with Fantasy Flight Games (stable of the MC collectible game), Isle of Man Film (makers of the MC movie - please forgive me for mentioning it) and Center of Gravity Games (a Swedish company which produced? - wanted to produce? - the 3rd edition MC with a radically different storyline and design, and might hold some of the rights). Any of those might possibly be offended if you wanted to revive the 1st edition since their vision of the MC universe is different. I don´t think they would feel exactly threatened by a fan project, though. If you ask all of them for permission, you´ll be on the safe side. On the other hand, it might be better to let sleeping dogs lie.
I love the idea of homebrew, and I´m looking forward to it! Though the general consensus seems to be that UWZ, in its current form, is a good and solid set of rules. Since there are significantly less players for the Warzone range than there are for tabletop games still in print, it probably would be best to stick to UWZ as standard, so as not to split the already meagre community.
That said, if you produce significant improvements, you can be sure that they will catch on like wildfire, since Warzone gamers tend to be an open-minded crowd. (At least that´s how they are in Germany.) And it is always interesting to look at different views. There´s some artwork from yours truly bouncing around in this forum that you´re free to use if you´re of a mind.
P.S. Correction: COGs license was revoked in Sep 2009 when they failed to deliver a finished product. The rights are currently held by Paradox Entertainment. Sorry!