Did I hear someone calling? :lol:
Here is a link to one on eBay, but you can find them at your local sewing machine repair shop for about the same price or less.
http://cgi.ebay.com/1-Home-Sewing-Ma...0V-FM15_W0QQit
I just looked up sewing machine motor in the search box on eBay and there was a slew of them listed. All you need is a universal styled motor and you are good to go.
According to the serial you gave you have a Singer 99 which 10,000 were allotted on Sept. 9, 1941. You did very good on the purchase of it, Model 99's are a nice machine and they sew very smoothly.
Billy