Use a signal generator with power amplifier and a transformer in reverse.
The signal generator generates the signal with the required frequency.
The power amplifier supplies the necessary power. An inexpensive 20 W mono amplifier should suffice.
The transformer is used to generate the 120 V from the amplifier's lower output voltage. The transformer needs to be matched to the output voltage of the amplifier. If, for example, the amplifier is rated at 20 W into 4 Ω it will supply ~ 9 V. The transformer would have to be a 9 V / 120 V transformer (or 120 V / 9 V in reverse).
When you use a sinusoidal signal that should be all.
If you use a square signal for higher efficiency, protective measures across the amplifier's output will be required to suppress the amplifier from the transformers flyback voltage when the current through the 9 V coil is turned off.