How do you fit a new gear to a Singer 427 sewing machine?

Hi
The bobbin mechanism has stopped working and I have discovered that the lower of the two nylon gears (top of machine) of my usually very trusty (bought new in 1973) Singer 427 has broken and missing teeth.
I have obtained a new gear from eBay but please could someone supply me with full instructions of how to fit it and do retiming?
Have had to resort to my great grandmother's 91 year old faithful hand Singer machine for the time being!
Thank you very much.
Regards
Chris