Try to keep your head up, this seems like an odd problem and uncommon to the yz450f (and 4 strokes in general). Try taking the top engine mounts off, they mount to the inside of the frame and to the head. That'll give you a little more room to work with. I usually end up seperating the gasket from the head cover, like you I find it next to impossible to get the cover off with the gasket attached. I'd think the screen on the petcock would be fine, but it's something that's relatively easy to eliminate as being an issue. If I were you, the first thing I'd do is I'd play with the idle adjustment screw. I'll take a look in my manual (2009) to see what it says as far as where it should be set - if it says anything at all. I'd try turning it in 3 or 4 rotations, something that will be noticeable and then with the bike running I'd turn it down to a normal setting. The next thing I'd do is I'd check the timing & clearances. Don't you just love the process of elimination ?