Header tank

first, it helps if you tell us the model of Coupe, but assuming it is a C model, while pulling the header is one of the WORSE jobs in a coupe, resealing the header is not that big of a deal and any decent A&P can do it.

I am assuming you have a later model Ercoupe. Mine is a 1946 415-C which comes with a 5 gal header tank and seeping fuel from somewhere above the tank on the co-pilot side.
Yes, the new tank for your plane may be an expensive part but at least they are in production. My 5 gal earlier model tank does not have a replacement due to not being in production anymore and either the old one has to be removed repaired and reinstalled or I should get a custom one rebuilt.
Imagine the pleasure of I am having! @#$%&

Good luck with your solution!

At my first annual last March they found a weeping rivet on the Alon's header tank (missed on the pre-purchase inspection and my pre-flights). After some back and forth the mechanics agreed to try Pro-Seal around the rivet before trying to remove and repair/replace the tank. So far that seems to be working. One of the wing tanks appears to have an epoxy layer around a rivet from some time past that is also working. The Pro-Seal or similar would be worth considering.

Finding these Ercoupe are definitely a "labor of love".

After getting my Coupe back from annual, I now find that the header tank seems to have a very very small drip/seep to it. I do not assume that anything was done at annual that would have caused this, but am going to take a closer look at it.

On first glance, it feels like the fuel may be seeping from corners/seems of tank. I have logs from the start and don't see that anything had ever been done to the header tank and it may just be simply leaking. I know that at least one of the wing tanks has been repaired.

With that said, I am open to whatever vast range of ideas this group may have with regard to resolving. Since owning (a year) I have approached maintenance items as a "do it right" and "don't get cheap on me now" type scenario, but the "new" tanks are ridiculously expensive so am open to ideas.