Whats the size of the screws for the carb. cap ? Is it good to replace them with SS with better heads ?

Allen head SS makes things nice. NO need to SHOVE & Twist, like with a Philips head. NO matter, SS or just an old bolt, aluminum is gonna want to grab it, so, yes, at Least oil, I use antisieze, on lug bolts on cars, on spark plug threads & on bolts that go in aluminum,

I somehow managed to keep the stock valve cover bolts. I went in there a bunch O times, LOADS more than our irascible Bill ever did. Of course, Suzuki installed "special" valves, rockers & adjusters in his, cuz his Never Needed adjusted,,
One thing to consider ,, gutless bolts in gutless heads,, which will crap outr first>  You go puttin SS bolts all up in that engine, dont forget, theyre still screwed into wussified aluminum,, dont be strippin stuff out just because youre not afraid youre gonna twist off a wussified stock bolt,
Remember to use a soft hand when snugging things down & RE snugging if it leaks beats the hell outta strippin something..