Konnichiwa lizmoran-san!
Usually, "Sayounara" or "Sayonara" is written in Hiragana. And there is not Kanji now. But old days, "Sayonara" was written in Kanji and Hiragana.
"Saraba" is a old word of "Sayonara" and was written in Kanji and Hiragana too. But usually Japanese people don't use this word "Saraba" in daily life now.
But I think that you should not use "Sayonara" in Kanji and Hiragana. Because tha Japanese don't use "Sayonara" in Kanji and Hiragana now.
NANGI