I don't think you will be able to accomplish this task using an ordinary VFD.
A synchronous motor does not rely on "electric slip" in order to produce torque whereas standard induction motors need "slip" to generate momentum.
If theoretically one could presume that both the sychronous and the induction motors' speed is proportional to the supplied frequency this is just about the only common characteristic.
Most "generic" synchronous motors ( except for the very small ones)need separate start-up excitation (due to the lack of "slip") and only dedicated Servo drives have this provision.
I think there are modern servo motors which started to bridge the gap between themselves and the simple and efficient induction motors, however, I don't think your old SEW is one of them.
I know you could perform servo applications using the PowerFlex 750 family VFDs, however, only when using specific, dedicated motors.