You know it's fried when it's nice and firm in the middle and crispy around the edges.
What Jon said about the brushes is oh so true. If the brushes stick and don't make contact or if they are dirty and grimy the motor wont run, or will have intermittent stoppages.
The best place to go to to find info about the potted motor rewire is here:
{
http://vssmb.blogspot.com/search/label/rewiring }
Rain is also a member here on QB.
And, here is a thread I did when I rewired my 15-91:
{
http://www.quiltingboard.com/vintage...r-t205360.html }
Several things I noticed:
One - You'll need new wiring blocks, the ones in your pics are trashed. Sew-Classic has the machine side separately and the cord side in a cord assembly.
Two - The splices in the motor need to be done differently and soldered. The way they are in your pics they will take up way too much space and will be very hard to insulate.
Three - Looks like you have / had some cleaning to do. You got enough lint and fuzz in there to stuff a cat with.
Joe