Welcome to the site. To start out, making the warrior lighter is pretty much a lost cause, most of that extra weight comes from the engine itself, which is about 2 times as heavy as the motor of a 400ex or z400. If you get new a-arms, plan on getting new front shocks too, because that extra leverage on stock shocks is going to make it ride too soft and squishy to get good handling out of it. My front suspension cost me about $700 for Works shocks and Burgard +2 a-arms. I wish I would have got +3 a-arms instead, because even with 1 inch offset wheels it's still not quite as wide as I would like. My rear end is just as wide as the front now, and all I have done is flipped the wheels.
But yes, there are quite a few options for increasing the warrior's displacement. The best bang for your buck is going to either be the Four Stroke Tech 446 kit or HotRods 400. Both are a bore/stroke combo, but the 446 is bigger both ways and requires more machine work and labor to install.
Stay away from the ebay widening kit. To start, the thing just looks like an accident waiting to happen. In addition, you can get a set of extended a-arms starting at around $350 that will be stronger, safer, have replacable ball joints, and increase your suspension travel a few inches. I didn't really expect it, but with my a-arms and shocks I have at least 2 more inches of front wheel travel than I had stock.
There's so many things available for a warrior that it would be overwhelming to post all the options in this thread. Take a look around the Elite, Images, and Quad of the month/pic of the month section and you should be able to get a pretty good idea of what all is out there.