Singerco.com has free downloads for earlier Singer machines in their "Resources" section. They may even have the correct manual for yours. The older 15 manuals should be close enough to get you by though.
I just checked. Someone here gave me a copy of the 15-91 service manual. There are some differences in the motor and possibly the upper threading but it should work for your purposes.
I'll send you a copy.
Rodney