The voltage that the motor needs has nothing to do with it.
You want a Mosfet that completely turns on with whatever gate voltage you have. Connect the source pin of the N-channel Mosfet to 0V.
An "ordinary" Mosfet needs a gate voltage of 10V. A "logic-level" Mosfet needs a gate voltage of 4.5V to 10V to fully turn on.
Does it not depend on the Drain voltage also? I've connected the IRF540 Drain to 5v, gate to 12v and I get it fully turned on. However if I connect Drain to 12v as well as the gate, it certainly doesn't switch completely on, I've got a reading of about 8v...