In the late 70s, a company in Denver came out with the Rare Earth Magnet Motor. It was about the size of an automotive alternator and produced more than 40 hp. They wanted to build a new tank that had little hydralics, an electric moved turrent and each idler on a tank would have one of these motors. They could build a tank using a Diesel Engine that was faster, lighter and lower. The weight savings could go to more ammo or thicker armor and about anything else the military was working on at the time. The Rare Earth Magnet Motor disappeared for more than 10 years. The problem was, it would have to be built from the ground up and the Abrams was nearing completion.
Funny, the only place the Rare Earth Magnet Motors were used was in the Aurora Super G HO slot car and for a few years, it was the car to beat until they also came out in the Tyco HO slot cars.
In the 90s, the motor resurfaced in Electric Vehicles like the GM. They also surfaced in the early 2000 in the Electric Bicycles.
It can still be done. While all the others require batteries as their primary power, something like a Tank can use an engine for the electric power. Look for something like that to go into production some day after they work out the bugs as a Light Tank or Personnel Carrier.