If I think calmly, I can come up with what to say.
If I think calmly, I can think of what to say.
If I think calmly, I can come up with what to say.
(落ち着いて考えればなんといえばいいか思いつく)
think calmlyが「落ち着いて考える」、think ofにもcome up withにも「思いつく」の意味があります。
I can think of what to say if I am calm, but I am not really used to chatting in person, so I cannot come up with English words or grammar and will hardly be able to speak.
(落ち着いてれば、いうことが思いつくんだよね。でも直接話すのには慣れてないから、英語の単語とか文法が思いつかなくなっちゃって、ほとんど話せなくなっちゃう。)
参考になれば幸いです。