you can also say مسرور instead of سعید
Yes, but مسرور feels like a bit of stretch to me for fun.
And مسرور is someone really into the thing., who doesn't care about the world when having fun.
And سعید is happy and fun.
But the sentence says "happy," not "fun." سعيد doesn't mean fun, only happy. So yes, we could substitute مسرور for سعيد just fine. If it isn't accepted, report it.