Sorry to say the foam rubber breaks down over time into a gummy mess. You'll need a good solvent to cut thought the mess (Toluene). Once you get it cleaned out you can use dense black wool yarn to replace it.
Get some contact adhesive and a cleaned out empty fingernail polish bottle with brush. Fill the bottle with some adhesive and thin it a bit so it's not stringy when you pull the brush out of the bottle.
Using the brush from the bottle paint the inner groove where the foam was located and before the glue dries press the wool yarn into the slot making sure not to compress it to much. You might find having a small wood stick cut to fit into the slot will help in guiding it in. That should do it!