I used to work on elevators, and sometimes had to have large brass ring gears made. I found that spin casting made the best ring gears. Like a bell, voids and uneven flows make bad gears.
You could make a mold spinner from an old axle end with a wheel on it, and spin it with a belt and slow speed motor.
Good luck to you, wear leathers and a full face mask - and the mold MUST be heat dried, as the SMALLEST bit of moisture WILL cause an EXPLOSION. That was the first thing I was taught when learning to pour babbit bearings.