increasing the inertia of your rotor is a perfectly acceptable way to model the flywheel. You will then have to determine how you want to control the motor. There are numerous control methods that work for asynchronous machines. The appropriate one is highly dependent on your technical requirements and what type of system the flywheel is connected to. For instance, microgrid vs full grid changes what assumptions you can make in your control algorithm. For a microgrid, your flywheel may be large enough to actually modify the system frequency and line voltage, whereas in a grid connected sccenario, you may be able to assume constant line voltage and frequency.
As an alternative, you can create a behavioral model that does not require an actual controller. This would be done by modelling your flywheel as a rotational inertia connected to a controllable torque source. You can then control how much torque is applied to the flywheel without needing a motor controller. Simply measure speed and multiply by torque to track your power, integrate to track your energy, and you have a model that you can push and pull energy into.