I would get price from local machine shops.
I have done up to 16" diameter on my G0759 mill using a rotary table and could go double that. Set up time is not too bad and the results were very good although you don't end up with the characteristic appearance of a turned surface. You could give the appearance of a turn part with careful application of emery paper by hand after the machining is complete. Hopefully you have a mill and can use this method.Do you have acces to a mill? A friend of mine will machine up to 40" flywheels on a turntable on his Bridgeport.
It is simple job to do on latheI do have two mills one cnc and one regular mill but no rotary table ,i thought about milling , but decided against that i only have one shot and i would hate to ruin a flywheel.
I put up a request for quote on another sight and i am having to put up a detailed drawing for quote i am working on that now here is a copy of that drawing .
Get your spare while you can; Same seller, must have a cast iron foundry. Pretty good price on the engine kit.i only have one shot and i would hate to ruin a flywheel.