Sorry for the huge gap in response time. Just got back to trying to work on this machine.
As for threading the bobbincase, I'm following the only instructions I could find online for a similar machine. The machine I am working has similar internals but looks older, with a solid, cast body.
I 'shopped this up from the instructions to try and show what I'm doing:
If you still think the spring needs replacing, any information on where to get one would greatly be appreciated.
Thanks again!
Tim