Takemichi can ask Naoto to search for some past events of that era. Takemichi can also go into the future and past anytime, so after going into the past, why doesn't he try to win some lottery or share market or something to get super-rich?
After becoming super-rich, he can control a lot of things. He can also save or hire the life of some highly-skilled boxer, martial artist, who then will guard him all the time. Right now, Toman and Mikey need the power to fight against Valhalla.