Buy the aftermarket item. It is only a plastic expension tank. I replaced mine with an ebay item.
Thank you. One last attempt to fix with second epoxy coat. If it fails, eBay it is!
Epoxy will not work as a repair. Heat cycling will crack it. There may be some other adhesive or coating that may work but I don't know what it is. A new tank is not very expensive.
Ordered one on eBay already. JBWeld radiator repair is the one I used. It reduced the leak by about 80%, but at the point of diminishing returns.
You might try talking to someone at the MB Classic Center to see if they might have any suggestions on where you might find your part.