I had the same problem with my 84 Big Red. The dampers were just completely toast. The entire fork setup on these two years is pretty bad. Once water and dirt gets in there, they just disintegrate. If you aren't looking to do a complete restoration, you can just put them back together as is. There really is so little travel up front anyway, and with the hard rear, I figured I wasn't loosing that much.
You can get the nylon bushings still for the inside of the fork sliders. If the chrome on your fork tubes is in good shape (mine was NOT), this will help freshen up the slider action. It was a bear getting the old nylon busings out though.
I thought that there may be a common automotive hood/trunk shock that would work, but didn't spend much time searching for it.
Good luck and post what you wind up doing.
- Frank
1984 200ES Big Red
1985 350X (x2)
1986 350X
1986 250SX
1984 Auto-X
1984 ATC70
1985 ATC70