So i was wondering if there was any way to change a cars position and or rotation using the world editor without resetting the cars momentum?
Rotation probably no because how would you imagine keeping the momentum after that, I don't think the game would be able to figure out in which direction it should be going. You can move the car and keep momentum without using the World Editor though, just pause and teleport it with free camera, it usually works.
This is true, when you use the free camera and teleport a car it keeps going wherever it was going before.