Warp has done a very nice Omega, though it isn't as long as you were hoping for. It's a resin kit, and assembled it's about 18 inches long. I have one, along with the Shadow Ship that Warp did. Neither are assembled though...too many other projects cooking.
Warp has no website, but head over to federationmodels.com and look under Model Kits, then select Warp. You'll be able to scroll down to the Omega.
Starship Modeler has also done a couple of articles as well that you may want to check out.
I've mailed Revell-Monogram many times trying to see if they were going to attempt to do more models in injection plastic, but they had no plans to pick up the license again. Sucks, but resin is about the only way to go.
>!<
RADAR
