The reddish stuff looks like old gas residue, but it could be some form of old seal-coat. The corks, just like cork carb floats, do have a seal-coating, and if ethanol-gas mixture has been used, that attacks the seal coat and ruins it, and the cork usually sinks. There are two kinds of natural cork. One is open-cell cork and the other is closed-cell cork. I replaced a tank cork with a wine bottle cork once without sealing it and it sunk within a week, so it was open-cell cork. I would clean your cork off good, sanding if necessary to get all that stuff off. Then seal-coat it with either clear, hot fuel proof model airplane “Dope”, or with super-glue.
Dave
