I know Smaug from the Michael Hague paintings mostly. In those, his head was REALLY lizard-like, except for the ears, they were like bats' ears. The best I can go for is to suggest something more similar to that.
When JRR made Smaug he usually used a serpentine body, but had a fairly stout neck and large head. The image of smaug on his horde has the longest neck of any of JRR's drawings and it wasn't very long. In his image of smaug been shot over Laketown, Smaug's neck and head are almost like and alliators.
Of course, what JRR wanted is not really that important. Ghost needs to find her Smaug.