Kudos on the rescue. I made my dad's bike that way. Chances are you could rebuild the derailleur easily enough, but check the pivot for play first. A lot of shifting issues with those are traced to pivot play. A new unit to replace that, actually better, would only be in the 25-30 dollar range. If you are putting a new crank and chain on it would be a minor price for a full package deal. It would save some issues later.