Leaking Fuel Tank

There should not be a fuel shut valve at the tank. Do you mean the fuel drain valve? Sounds like you may just need to reseal the threads. I would also check the flange to make sure that has not failed. If it was not flanged correctly at the connection from the tube to the tank, the fuel tube can become cracked and start leaking.

Worse case, to prepare yourself, will be to reseal the tank which will require drilling out a couple hundred rivets. There is a write up on this on the ercoupe info website.