I would try Fabreeze. If you weren't afraid of damaging the lining, you could spray it directly on. Otherwise you might try spraying a paper towel with Fabreeze and closing it into the case for awhile. This stuff is awesome for removing odors from everything I've tried it on.

Try Irish Spring soap, in the box. You can also use Safeguard, as recommended by Fred, The FW repairman from Texas, who was at Paducah last spring. He was well worth the entire cost of admission, just hearing his wisdom concerning FWs. If you are in Paducah in April, be sure to look him up. He has lots of parts and books with him.

My case was stinky too, so I use a soft case for my featherweight. I think they priced it so low at the thrift shop because the case was stinky and the handle was broken. I got lucky that day - I got it for $27.00