Cummins N-14 has low power under a load?

The N-14 series is a good motor, with a fairly wide powerband. If the motor seems underpowered, it may be because of the tractor specs - the N14 series ranged from 370 to either 500 or 525 horsepower, and torque output was considerably different as you went through the horsepower range.

Again, you may want to look at the overall tractor specs. If you're running in mountains, and your tractor runs something like 3.55, 3.58, 3.42, or 3.36 rear end gears, I'd look to that more as the source of your power problems rather than the motor.