If you're smarter than (or think you're smarter than) a 5th grader you can do the math and calculate the (rotational) energy in a spinning flywheel disc = 1/2 x rotational inertia x angular speed-squared. Use say 2000 RPM, 1" diameter brass disk, 1/2" thick, do the scale conversion. Then calculate the ginormous capacitance to store an equivalent amount of (electrical) energy = 1/2 x Capacitance x Voltage-squared.
The problem is compounded since the capacitor is holding the energy is electrical form and must be converted to mechanical energy by the motor. A can DC motor as used in O-gauge engines might be running at, say, 33% conversion efficiency so now whatever capacitor size you calculated must be multiplied by 3.
Of course if you don't have the ability to attach or augment the size of the existing flywheel then you're stuck. But I think a chunk of brass will be cheaper than a capacitor storing an equivalent amount of energy.
