Center crank seal is very similar to a normal deep groove ball bearing rubber contact seal. The difference is that it has a spring like an oil seal on it's inner lip and this actually seals on the crank web, not the inner race of the bearing.

I do know that people here in Bodge-land have actually fixed these things. I won't go into their methods as the last bloke I spoke to who was bleeting on about how easy the fix was then went and seized it