The most difficult problem(s) you will have is removing the pinion gear and reinstalling a pinion gear that fits the P60 transmission. Get a pinion puller similar to one at this link. Other designs are also available at most hobby shops:
http://www.toysonics.com/walkera-uni...-g1001002.html
When reinstalling the P60 pinion back on the shaft, you will need an arbor press. Since the motor shaft is long and thin, unless the shaft is held perfectly perpendicular to the press ram, you run the risk of bending the motor shaft, rendering the motor useless. You may want to manufacture a fixture that will hold the F/P motor perfectly square while pressing the new pinion on the shaft.