How can I replace the Top Vertical Shaft Bevel Gear for SINGER MODEL 248?

Hi
The part holding the bobbin case in my Singer 248 stop moving. So the machine dead. ( can't take up the bobbin thread and sew ), the needle is moving up and down when i turn the hand wheel.
Open the machine and find the top vertical shaft's plastic gears are rotten. it need to be replaced.
Would someone give me instruction in how to replace it ?
Thanks you.
Mitchie