Since the E-1 engine is closely related (a licensed copy, essentially) to early British MG four-cylinder engines, I had heard and assumed to be true that a valve cover gasket from an early 1960s MG would fit.
Based on this hypothesis I ordered some parts from Moss Motors. I bought a simple cork valve cover gasket for a 1960s MGA/MGB, which is part #296-310 and was only $2.45. I also bought a premium silicone gasket set which included the valve cover gasket and a pair of inspection cover gaskets. That was Moss part #296-425 and was $19.95.
The good news: The silicone inspection cover gaskets were a great fit. I matched them up with the inspection plates and bolted them back onto the engine.
The bad news: neither of the valve cover gaskets were a great fit. Both lacked holes for the six screws that hold down the valve cover. I know E-1 engines from earlier 320s did not have those six mounting screws, so perhaps these gaskets will fit those–but I can’t speak to that. Even so, I noticed that the cork gasket was larger than the silicone one, meaning the silicone gasket would have to stretch to fit the valve cover. Perhaps that is by design, but it is worth noting.
Ultimately I decided to use the valve cover gasket from the Victor Reinz J13 engine gasket set I was able to acquire. That gasket has the holes for the mounting screws.
I have two J13 gasket sets on hand, but since they are getting more difficult to find, I decided to make a CAD dimension drawing of the gasket for posterity. Below are some pictures of the drawn gasket (in white) that I printed out and taped together to be full-sized.
And for anyone interested, here is a PDF measure drawing of the gasket.