Found this link to a service manual you can print out for it:
http://parts.singerco.com/IPsvcManua...57_651_658.pdf
Didn't see a threading diagram in it, though.
Also found this old eBay listing for the head so you can get an idea of pricing:
http://cgi.ebay.com/sewing-machine-S...-/180638893354
Also found this place that sells industrial sewing machines. You might want to call or email to ask about threading:
http://www.industrialsewmachine.com/...hures/211g.htm