If I could change places with someone for a single day, I would choose Kohshi Inaba, (who is) the vocalist with/of B'z, a Japanese rock duo. ["With" is usually used for a larger group of people. More natural: "... the vocalist in the Japanese rock duo B'z."]

The reason (why/that) I'd choose him is that he is the singer I admire the most highly. [More natural: I would choose him because I admire him more than any other singer.]

I think no one else can sing a rock tune in so cool a way as he can. [More natural: "I think no one else can sing a rock tune in such a cool way."]

I'm sure it feels so great to sing in front of great many people in a voice as strong and powerful as his.

So, if I could be him for one day, I would perform a concert at a big place, such as Budokan or Tokyo Dome, and sing and shout until I'd lose my voice. [The verb "play" is only used for musical instruments, not for the voice.]

If I could change places with one person for a single day, I would choose Kohshi Inaba, who is the vocalist with B'z, a Japanese rock duo. (or a Japanese Rock duo called B'z.)

The reason why I'd choose him is that he is the singer I most admire. most highly.

I think no other one no one else can sing a rock tune in so cool a way like as him. (or as he does. see below.)

I'm sure it feels so great to sing in front of a great (or so, to avoid using great twice) many people in a strong and powerful voice like him (ok in speech, but like he does is better in formal writing.)

So, if I could be him one day, I would play a concert at a big place (or venue), such as the Budokan or the Tokyo Dome, and would sing and shout until I'd lost my voice.