I've never seen any kanji except 村上 for むらかみ. so it should be the most common kanji in Japan. as for ゆみこ, there's a lot of variations that could be such as 由美子、裕美子、祐美子、弓子、友実子... I can tell you that 子 is always used for こ when it comes to Japanese girl's name ending with こ. ex) 由紀子(Tsukiji)、幸子(sachiko)、恵子(keiko)、陽子(yoko) .. etc hope it helps!

If you are talking about your friend whose name is むらかみゆみこ, I think the best way would be asking to her directly. We usually ask which kanji is used for the person whom we talk with or for their family members because there are many people who doesn't use "the common way".

In my opinion, "由美子" is the most common way. And other example, I've seen "弓子” and "裕美子". "村上" is the most common way.