As Cherokee99 said, use penetrating oil to try and loosen it up. Then you can get an easy out or other brand or extractor. Drill a hole in screw, insert the extractor and remove it. If that doesn't work, drill it out and re-thread it.

 

My husband and I just rebuilt the carb on our 2000 Grizzly 600. We did have to remove the front plastic and the fuel tank to get to it, but it really wasn't difficult. We also had some issues with the screws and we needed to replace ours as well.

I really don't think it's necessary to go to a dealer for new screws. If you're going to replace them, we really recommend allen head cap screws. You can pick them up at any hardware store for a pretty reasonable price. We got ours from Lowe's but I'm sure you could also get them at HomeDepot, Westlake, ACE, etc.

If you're interested, you can read more about our experience here.
-Beth