I'd have to replace the front tension, which most experienced folks say is tricky. It has to go back into the front plate in a particular order. And there is a rusted tension spring on the thread take up bar. And, for some reason, while looking at it today, the bobbin race broke.
I find it odd that I can't swap parts on the two heads. The bobbin and bobbin holder fit in the newer head, but it's not picking up the thread. I should post a picture of the newer head too:
[ATTACH=CONFIG]519624[/ATTACH][ATTACH=CONFIG]519627[/ATTACH][ATTACH=CONFIG]519628[/ATTACH]