please remove.

 

There is a drain bolt in the bottom of the gas tank. Raise the car so that bolt is the lowest point, and drain the tank. You may get some of the rust out that way.

I'd try just changing the filter sock on the bottom of the pump. If the pump works OK with the tank full, it is still a good pump. Make sure all the fittings on the inlet of the pump are secure so the pump doesn't start sucking air when the tank is half full.

It is very simple to drop the tank since (unlike most American cars) you can drain it first, and disconnect the fittings on top from inside the car. Perhaps worst case replace the gas tank entirely.

 

I was saying to replace the sock again because your "new" one could be full of rust now.

You should replace the cap for sure, but I doubt it will help any.