I would liked to upgrade my single speed pool pump motor to a variable speed motor. I currently have a Century Centurion 1.5hp single speed pump motor. I would like to retrofit the current motor with a Century variable speed motor. Here is the current motor info: CAT: B128, Serial # 23916J2, Part: 7-164303-23, Type: CX, Frame: M56J. Hopefully that is enough info that you will be able to recommend a variable speed replacement.
The replacement variable speed motor that replaces your B128 motor model is the ECM16CU that you can view by clicking here. Be sure to replace the seals and gaskets as well and in order to cross-reference those you will need to know the model name or number of the front pump wet end housing that the motor attaches to.