Fushi no Kami: Henkyou kara Hajimeru Bunmei Saiseiki

Ash is an 8-year-old boy with apparent past-life memories, who lives in a remote deserted village. As he remembers leading a bountiful life, he cannot bear living in a world akin to the Dark Ages, where not even the internal combustion engine exists. For that reason, he needs to consult books containing knowledge from a highly developed ancient culture, that is said to have existed far in the past. This is only the beginning of story about a young boy who sets out to revolutionize the world in order to rebuild civilization and achieve his ideal life! (Source: J-Novel Club)