Speedpro1 is correct, the current rating needs to be pretty high! Even a 17.5T motor on 2s LiPo will have a stall current close to 200A, and there are plenty of classes using motors having much higher power. But the current will only last a second or two, at the start of spool-up.

(An observation: The ESC actually doesn't need to be a speed control; it just needs to commutate the coils based on the sensor signals and a single enable signal. That doesn't even require a microprocessor! But I don't know if that would save any money, considering how cheap off-the-shelf ESCs are these days.)

I know of no flywheels that are both specifically made for our motors and affordable. For production, the cheapest solution that I can think of is to use a cast-iron V-pulley of an appropriate size (2.5" or so?). But it would have a little more surface area, and therefore drag, than necessary. If the losses are compensated for, that shouldn't be a big deal.

I intend to try the flywheel below; it looks like it's from a reel-to-reel tape deck capstan, and weighs 458g. It cost $15 from the evil auction site. It is used, so it would not be a solution to a production run.