How ever I think the chances of getting a Death City Rock Group together are fairly slim. The only other real candidate for the band would be lord death since he can play Mandolin and that might translate into guitar skills. But after that, all you've got is blind hope.
I'd probably cast him as drums. Him being able to wield the Kasuragama as a two handed weapon I think makes him the best suited for the sticks. I know Kids all fancy with two guns though and he's got the twirling down, but he does more shooting then hitting. And then Kilik would be bass. Because as South Park so offensively put it:
Well, Cartman originally said it for Token, but Kilik is the only Black Kid in Shibushen I've seen, so I think it still fits. I dunno why, but I think I'd like Spirit as my vocalist. He doesn't seem like the singing type, but I think the fact that his voice actor Vic Mignogna had played some serious heavy weight characters in the past means he has gruff tones in their somewhere as well.