After exiting his country, Lei Feng climbed the highest mountain, explored the plain, survived in the wilderness, weirdly, no animals was present, only some insects, but even without the presence of animals, they didn't multiply crazily.
Then after coming to some new country, and as he already learned practically every language, so books are readable. Learned more about science, and then two hundred years passed. Having a photographic memory he finally learned how to create a nuclear fission, even so he would not touch something so dangerous, only in so much lonely time, if he didn't found something to do he would become crazy.
Then three hundred years later after traversing the sea, and explored another continent, he finally learned how to do nuclear fusion. Transcending the actual science technology. For creating something so high-tech, the power of the sun, he did learned many, many, many other area of science, from metallurgy to applications of the science, he can't even be called a genius, but a monster.
He used new metal he created until getting something as solid as a legendary metal like mithril. Then using some specific and detailed science, he finally created the generator of nuclear fusion.
Then a hundred years passed again, he created the small generator that is only taking three meter wide, using this generator, he modified some plane to become a nuclear fusion airplane, yeah that sound cool, even more, that energy will never depleted.
Then again two thousand years later, he finally created the thumbs sized fusion nuclear generator that can power up his power armor that he created by himself. As an old kid he really loved mecha, so he created a power armor that can envelope his entire body, even if it's weighing a ton, from metal that he created and upgraded again and again, he can easily lift it because he now can lift up two and more truck, one for each arm, but he could surely lift ten more.
That power armor is only a prototype, what Lei Feng aim to create, is a mecha, that can fly into space, then he could have explored new world in space, but even so he is barely coping with his loneliness, and hoping to meet alien, even if that is totally impossible from the probability, he could have at least tried, using millions of years.
But at the end, he couldn't really quit his planet, so for a hundred more or less years he just focused on creating a mecha, with not only one generator, but with twenty more generator spread evenly in that mecha that height three meter, even using a new technology he just learned, the nano machine that could automatically repair damaged part.
Using the rocket from his back, feet and hand, he could travel with enough acceleration to at least ten thousand kilometres per second, so in half a minute he could fly to the moon, and he also counted, and evidently tested. By the way, there is absolutely nothing interesting except the hydrogen that could refill his nuclear generator, so he invented the hydrogen collector, using a small box of ten centimetres to collect and compress a thousand of ton of hydrogen that could last him like forever.