I Was Supposed to Be a North American Detective, So What’s with the Astartes?

Li Wei transmigrated to America and became an ordinary patrol officer. In the land of the free, being a patrol officer is hardly a good job. You could get killed! As Li Wei wondered whether to find another line of work, a panel suddenly appeared: [Welcome to Warhammer 40K] [In this world, chainswords dance alongside psychic powers. The Chaos Gods of the Warp and alien races threaten humanity, while the Imperium is already tottering under the weight of rebellion and decay!] “Uh... This sounds so chuunibyou. Not very smart, either. Can I get a different one?” [Alert! High-level threat detected! The wicked Daemons of Slaanesh have somehow gained followers aboard the starship. Their utterly depraved plan is to steal the Space Marines’ gene-seed!] [As a warrior of the Emperor, sworn to honor and loyalty, go defeat her!] [Mission reward: 1 gene-enhancement serum, 1 implanted organ] Looking at the American sweetheart pressed against him, Li Wei swallowed hard. “Wait, now that you mention it...” … [Secondary heart, reinforced bones, enhanced muscles, modified blood...] Every time he completed a mission, he received an opportunity to modify his body. When the nineteenth modification was complete, Li Wei discovered that he had become a god! “America, you’re a mess. Time for you to get a new sun!” P.S. America, I alone wield magic, a mismatched System.