I mean, he's literally just an unmodified Noise Marine... but I think it's an effective "conversion", or... more like a proxy, I guess? I imagine he bashes people with his guitar and shoots them with the bolt weapon part, rather than actually carrying a Crozius and pistol. I dunno, it was a cool concept, and I guess it worked, because ain't nobody complained yet, and he's sparked a whole Word Bearers army (one CSM squad with Shadowspear's loadout minus one point short of 2k at the moment) so I guess he's done his job.
"All power demands sacrifice and pain. The universe rewards those willing to spill their life's blood for the promise of power." - Sindri Myr
"There is no meaning, no purpose. We murder, we kill. It is mindless savagery, this UNIVERSE is mindless!" - Azariah Kyras