I would place the machine (and case, if there is one) inside a large garbage bag along with a pair of OdorEaters shoe inserts -- the ones with charcoal in them. Two pair if you can afford it. Tie the bag and leave it for two weeks. Charcoal absorbs odors, and this is a non-messy way to use it.
Edit: I should add this is how I got rid of the musty smell in my featherweight's case -- after trying many other methods. I keep one of the OdorEaters in there all the time now, as it also cushions the foot from the rest of the machine.