If you're replacing the hop-up in your KWA, you can't replace it with anything other than the KWA one.
If it's any other M4 AEG, I liked my Madbull Ultimate HUU. I've heard great things about the new Modify one.
http://www.precisionairsoft.com/PCT-6-0 ... ct247k.htmI'd wait for that barrel to get back in stock and get that one. PCT (AKA Dees) barrels are some of the best I've used. I own 3 of them, and plan on putting more in my other blasters as money permits.
That's a G&P Defender, not G&G. I'm not sure they ever sold that lower in a kit. Just as a part of that AEG.