The motor size question does not require a formula, you simply need the same horsepower motor.  The  speed-reduction drive is a constant horsepower device:  horsepower "in"  equals horsepower "out," minus some small losses of typically 2% to 5%.  To  drive your mixer at the same speed, 800 rpm, with the same torque, you need the  same power.  Under normal conditions, an equal-size electric motor designed  for use with a VFD, should deliver the amount of power your mixer  needs.