How do i flush a in built boat fuel tank totally clean?

OK heres my problem, just had a beautiful new alloy boat built with a 100lt fuel tank built in, used my old engine on it for 3hrs which was pre mix 2 stroke so have both oil and fuel in the tank! Now have upgraded engine. My new engine is post mix 2 stroke!!!! so how do i completely flush the old fuel out and clean tank of all oil before new engine can be safely used and tank filled with strait fuel?