The closest I have come to a complete disassebly is a 66 on the Vintage Singer forum. Personally I have never gone that far, luckily you can get most things up to standard with out. However, if you insist it is possible. Some models may need speical tools and access to a metal workshop could be handy.
Kerosene on all joints, gears and hinges help dissolve grime and flush it out. It is importat to keep on oiling and runnng the machine now and then, kerosene will evaporate. If you go for a complete disassembly it's not that important though.
My black cast iron 201 from 1950 has tapped in spool pins. I know, because one was missing and one bent. Once in place they stick rather well to the body. I think my beige 1955 aluminium 201 has threaded screw-in spool pins.