Nearly all early Fanuc AC spindle motors have a 256 tooth gear on the end. This is read by a quadrature sensor that generates a small AC signal riding on a 2.5 Volt DC signal. It is squared up inside the drive to produce 5 V square waves. The motor is a simple induction motor and runs around 200 volts at various frequencies. It should run quite well with a generic VFD, even without feedback (sensorless vector). The kicker is usually the orient system. It complicates everything.
Why change out the drive? Parts are out there.