The trouble is that if there is a large enough gap for things to flow out, other things, like dirt and grit can get in. I suspect that you are right that the shaft (#30?) has worn a bit at the seal area. Can you get the part? If not, can you, or can you get a nearby machinist to, lay down a weld bead, or sleeve it, and machine it back down to size?
If you don't use it often, could you put a rubber sleeve, or a few layers of self fusing rubber around the open area adjacent to the bearing and cap it with a hose clamp to make an "outer seal"?
I wouldn't go with grease as the chain and bearings is designed to be sprayed with oil.
All the best,
Peter