I am looking for a motor to replace a FRS 550 PH that I took out of a heavy duty sears 1/2 in drill. The motor is burned up.

You can buy a 550 motor for about 5 -10 $$ remove the pinion gear from the old one and put it on th new one
http://www.trossenrobotics.com/store/p/5152-RS2-550-Motor-12V.aspx?feed=Froogle

My Mighty Mule 500 was used a few times daily for almost 3 years on a small gate. The fuse started blowing randomly caused by the brushes being completely gone.

Replacing the motor with the one discussed above took a couple of hours and a lot of patience. First, it was helpful to unscrew the arm a little (making it longer) so the tiny switch inside didn't get broken when the arm was pulled apart. Second, removing the 5 gearbox cover screws and taking a couple of the gears out to get to the two screws holding the motor was a small challenge but not too bad. Third, the special TVS diode soldered to the motor tested open with my meter but I later found out this was normal because it is designed for suppressing lightning strikes.

I've had several Mighty Mules and the 500 was noticeably slower than the others. I thought this must have been because it has a longer arm for larger gates. The replacement motor must run at a higher RPM though, because the gate now opens as fast as it did with older Mighty Mule models.