How important is it for the .1uf capacitor to be film? What can I substitute in there for the film cap?
It doesn't matter that much. A different cap might give you a subtle tone change, but I use the standard teeny blue caps and they sound fine.
What else do you have?
You should be better substituting other types of caps, but IMO films sound better than something like ceramic/tant/electrolytic/etc. They all have their advantages and disadvantages, but if all you're missing is that cap, stick something else there for the time being.