You can replace the bleeder valve. I have pulled one apart on a bad tank out of curiosity and it's not too complex. Here's a thread that I've read before:
My first Question, Bleeder Valve?
I would recommend replacing the entire tank unless you know that it has been done in the last several years. The tank will wear out eventually. Why not do it now. There are lots of complaints about aftermarket tanks. Many of them are flawed right out of the box. Best to go with Porsche brand even though it costs (much) more.
The easiest way to replace the tank:
DIY Coolant Tank Replacement in Under 2 Hours