"We Strengthened the structure but not changing on vibration" What exactly was done?
It is difficult (especially without experience) to modify a structure without understanding how it is moving/deflecting. An operating deflection shape (ODS) test with two accelerometers, or an impulse-response test with instrumented hammer and accelerometer are two methods to quantify structural deflection.
I could tell stories of how this exact problem was solved several times. If you want to try an experiment to add mass to detune the structure, then here is a practical method that has worked:
Remove the two motor lift-lug eye-bolts and get two bolts with same threads of suitable length
Make two steel weights (start with about 25-lbs each and about 50-lbs maximum) from steel blocks or plates (less than 12-inches square)
Make hole in center of each weight (drill or torch-cut) for bolt
Bolt each weight tight to motor case at the two lift-lug locations
Run motor with base bolts tight with no soft foot present. Note that shims should be snug at all four base locations when base bolts are loose to minimize soft foot.
If the vibration level at 2x line frequency (100 Hz) is substantially reduced, then a natural frequency was present and controlled. Either leave weights in place, with appropriate engineering review, or remove weights and evaluate other vibration control methods.
If you try this, then please provide before and after vibration measurements.