I'm a librarian and some years ago, someone returned books that had been "skunked." The kitty litter method worked well - empty a bag of cheap, clay-based litter into a plastic garbage bag. Lay the item on top (no need to submerge). Close bag very tightly, and allow to sit for a couple of weeks (a few days is not enough). It worked for us. Good luck!

Charcoal briquettes work wonders. Put machine into plastic bag or big tote and lay the charcoal around it for at least a week.

Does anyone know of a repair person who is good with Featherweights in the Ann Arbor/Ypsilanti area? Good luck with yours.

A lot of the odor may be coming from the felt on the bottom plate. Have you taken the bottom plate off and checked the attached felt for odor?

Part of the odor may be coming from the felt on the bottom plate. The felt can be changed if needed.

Did you remove the bottom of the machine? There is a screw, undo it, remove the bottom, there is a felt liner that needs to be replace if it stinks.

Featherweight repair
 

The videos from The Singer Featherweight shop can help you do many repairs yourself. You might be able to do it yourself. They are excellent and their service and products are great. https://singer-featherweight.com/

For the case, you might try putting some whole cloves in a small net bag or something similar that breathes.

There is a replaceable felt pad inside the bottom of the machine. It collects excess oil. It can become stinky.