Deck is far from perfect, but considering you claim to not be good at deck-building, it’s pretty damn good. I don’t think it’s possible to reach S with a magmar deck unless it’s refined which your isn’t and won’t be unless you get some of the RotB cards so might as well not bother to alter it too much and focus on gathering resources to craft more cards in general.
But if I had to make suggestions to the deck while keeping the original idea of it in mind, ie it being aggressive but not full retard aggro, I’d do something like removing one plasma storm and earth sphere in favor of sojourner and warlock for some extra consistency. Alternatively you can switch to a more lower curve aggressive version which is currently popular but like mentioned, you need some more RotB cards for it or you can go for a more midrange-ish version which runs stuff like flash and sunsteels. Not sure how good it is in the current meta though.
It all really depends on what your goal is with the deck.
edit: Yeah, don’t know how I managed to miss young silithar. They’re so good that I just imply they are in every magmar deck by default. Definitely find some room for it.