As luck would have it, Jon Goodman had a promotion on his seal kit for Nikon FG and EM. I bought two of them and replace the seals on both of my FGs. With his instructions it was an easy job. I ran a test roll through the first one late last week and found no indication of light leaks. The second camera has a roll of Tri-X in it now.

Interestingly, I just checked the seals on my Minolta X570 and XE7 and found that they were made of felt which does not degrade the way foam does. I wonder why Nikon didn't use felt.