It seems that it should be spongy, like the repro front parking lense gaskets.

Try this: Gently scrape off whatevershit you can without damaging the gaskets. Put them in a pot of boiling water and spike the water with a healthy dose of liquid dishwashing soap. Brew them for a while, and the gaskets may spring back to life. I did it with parking lense cover gaskets with exceptional results.

An alternative would be to back up the body side of the old gasket with dumdum. Its slang name is strip caulk. Common sense is the measure as to how thick the dumdum should be.