Stiff might not be bad. Gooey is bad. Clean them up.
I think someone else mentioned to carefully mark their original positions, when removed. (Left front, left rear, right front, right rear), so they can go back exactly as before, and look for, retain, and reinstall the tiny washers that were installed. There is one on the back and front of each roller. A small drop of sewing machine oil goes each capstan shaft before reassembly.
You can use some fine sandpaper (400 grit), to expose fresh rubber-just be sparing with the paper-only need to remove just enough......
You don't want to alter the profile of the rollers. Wrap the paper around the roller, and use your fingers to spin the roller in the sandpaper loop. Another method is placing a sheet of the paper on a flat hard surface, and spinning the roller on the paper, always mindful to keep it aligned such that you don't create flat spots or any irregularities to the profile of the roller.
Nice deck-and like you did back in it's day, I lusted for machines like yours, which I was unable to afford then.