Reincarnated As The Villainess's Best Friend - Chapter 17: Here Comes The Tournament

A scientific genius named Thomas Harrier was transmigrated to an imperial prince body after a car accident that took his life. He found out that he is in a world similar to our own with a timeline akin to early twentieth century. The body that he is occupying was the imperial prince of a failing empire who suddenly rose to power after a successful assassination attempt of his father. Unaware that the attack is just the beginning of what is to come. He faces many challenges throughout. Will Thomas Harrier be able to live his second life in peace while running an empire and getting it back to its former glory? Or will he succumb into a conspiracy lurking within the country's borders?

Expect some:

Cold war era techs.

Super battleship with advanced electronics, propulsion, and fire control systems?
Helicopters?
Missiles?
Modern medicines and equipment?
AC-130?
A nuke?
Challenger or Abrams or Leopard?
F-4 Phantom or MIG-21?
Colored TV's.

Everything!!!

PS. Cover not mine, I just found it on google.

The celestial celestial beings who lived for more than 100,000 years were born and reborn in a small monk in a human world.
His motto: “It’s not as good as a medicinal medicine for ten years of hard work.” “Even if you practice a copper-iron bone, it’s not enough for my soldier’s attack.”

- Description from Novelupdates

Traveling through the world of pirates, becoming Luffy, and possessing cheats in teaching space, it is natural to make changes.

When Chambord landed on the island, the pacifist, the general Huang Yuan, appeared, and all the supernovas fell into despair except the hat group.

Seeing Zoro sweeping the world with a single sword, Sanji the Demon Kazekaku, and Luffy blasting away the oracle with his four-speed karma fist gun, the supernovas were stunned.

“Is this a supernova? I’m afraid it’s not the imperial group!”

During the war on the top, the hat regiment descended with the momentum like a royal regiment.

On Ghost Island, Kaido and Big Mom satirized: “Hey, boy, what are you going to become? Say it again in front of us!”

Luffy opened five gears furiously, his fist was as huge as a ghost island! A five-speed Yehuo fist gun! Party Kaido and Big Mom!

Luffy landed, the black stand-up collar cloak fluttered, put on the helmet, and whispered domineeringly:

“I’m Monkey d Luffy, the man who wants to surpass you and become the One Piece!”

- Description from novelbuddy

The protagonist transmigrates into a novel and gains a system where they grow stronger by simply writing in a diary.July 7th, sunny: “The boss’s outfit is hideous again. Wouldn’t it be better to wear something cooler?”July 7th, evening: “I’m about to have my engagement called off. So excited! How should I act?”July 7th, evening: “Lin Qingyue, the female lead, is probably going to drown her sorrows in alcohol.”The protagonist thought it would be easy—just jot down daily musings, record life’s events, and earn rewards. All while playing the part of a villainous, bootlicking second-generation rich kid, moving along with the original plot.But unexpectedly, the female leads gain access to copies of the diary entries.One by one, they stop following the script, and the plot spirals completely out of control.July 10th: “The female boss is an idiot—no one would want her.”Soon after, the protagonist gets punished at work, all because they entered the office with their left foot first.